Just tried it, but it appears to fail if I include it in the Profile JVM settings. It is ignored if I add it to the .json file arguments. Here's the crash log:
[spoiler=Log]
[14:10:16 INFO]: Minecraft Launcher 1.3.8 (through bootstrap 5) started on windows...
[14:10:16 INFO]: Refreshing local version list...
[14:10:16 INFO]: Current time is Jan 18, 2014 2:10:16 PM
[14:10:16 INFO]: System.getProperty('os.name') == 'Windows 8'
[14:10:16 INFO]: System.getProperty('os.version') == '6.2'
[14:10:16 INFO]: System.getProperty('os.arch') == 'amd64'
[14:10:16 INFO]: System.getProperty('java.version') == '1.7.0_25'
[14:10:16 INFO]: System.getProperty('java.vendor') == 'Oracle Corporation'
[14:10:16 INFO]: System.getProperty('sun.arch.data.model') == '64'
[14:10:16 INFO]: Refreshing remote version list...
[14:10:16 INFO]: Refresh complete.
[14:10:16 INFO]: Loaded 6 profile(s); selected 'Test'
[14:10:16 INFO]: Refreshing auth...
[14:10:16 INFO]: Logging in with access token
[14:10:24 INFO]: Getting syncinfo for selected version
[14:10:24 INFO]: Queueing library & version downloads
[14:10:24 INFO]: Download job 'Version & Libraries' started (16 threads, 11 files)
[14:10:24 INFO]: Attempting to download C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\java\jinput\jinput-platform\2.0.5\jinput-platform-2.0.5-natives-windows.jar for job 'Version & Libraries'... (try 0)
[14:10:24 INFO]: Attempting to download C:\Users\Wes\AppData\Roaming\.minecraft\libraries\org\lwjgl\lwjgl\lwjgl\2.9.0\lwjgl-2.9.0.jar for job 'Version & Libraries'... (try 0)
[14:10:24 INFO]: Attempting to download C:\Users\Wes\AppData\Roaming\.minecraft\libraries\org\lwjgl\lwjgl\lwjgl_util\2.9.0\lwjgl_util-2.9.0.jar for job 'Version & Libraries'... (try 0)
[14:10:24 INFO]: Attempting to download C:\Users\Wes\AppData\Roaming\.minecraft\libraries\org\ow2\asm\asm-all\4.1\asm-all-4.1.jar for job 'Version & Libraries'... (try 0)
[14:10:24 INFO]: Attempting to download C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\sf\jopt-simple\jopt-simple\4.5\jopt-simple-4.5.jar for job 'Version & Libraries'... (try 0)
[14:10:24 INFO]: Attempting to download C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\java\jutils\jutils\1.0.0\jutils-1.0.0.jar for job 'Version & Libraries'... (try 0)
[14:10:24 INFO]: Attempting to download C:\Users\Wes\AppData\Roaming\.minecraft\libraries\org\lwjgl\lwjgl\lwjgl-platform\2.9.0\lwjgl-platform-2.9.0-natives-windows.jar for job 'Version & Libraries'... (try 0)
[14:10:24 INFO]: Attempting to download C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\java\jinput\jinput-platform\2.0.5\jinput-platform-2.0.5-natives-windows.jar for job 'Version & Libraries'... (try 0)
[14:10:24 INFO]: Attempting to download C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\minecraft\launchwrapper\1.5\launchwrapper-1.5.jar for job 'Version & Libraries'... (try 0)
[14:10:24 INFO]: Attempting to download C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\java\jinput\jinput\2.0.5\jinput-2.0.5.jar for job 'Version & Libraries'... (try 0)
[14:10:24 INFO]: Attempting to download C:\Users\Wes\AppData\Roaming\.minecraft\versions\1.4.7-Forge6.6.2.534\1.4.7-Forge6.6.2.534.jar for job 'Version & Libraries'... (try 0)
[14:10:25 INFO]: Download job 'Resources' skipped as there are no files to download
[14:10:25 INFO]: Job 'Resources' finished successfully (took 0:00:00.000)
[14:10:25 INFO]: Finished downloading C:\Users\Wes\AppData\Roaming\.minecraft\versions\1.4.7-Forge6.6.2.534\1.4.7-Forge6.6.2.534.jar for job 'Version & Libraries': Couldn't connect to server (responded with 403) but have local file, assuming it's good
[14:10:25 INFO]: Finished downloading C:\Users\Wes\AppData\Roaming\.minecraft\libraries\org\lwjgl\lwjgl\lwjgl-platform\2.9.0\lwjgl-platform-2.9.0-natives-windows.jar for job 'Version & Libraries': Local file matches local checksum, using that
[14:10:25 INFO]: Finished downloading C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\minecraft\launchwrapper\1.5\launchwrapper-1.5.jar for job 'Version & Libraries': Local file matches local checksum, using that
[14:10:25 INFO]: Finished downloading C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\java\jinput\jinput\2.0.5\jinput-2.0.5.jar for job 'Version & Libraries': Local file matches local checksum, using that
[14:10:25 INFO]: Finished downloading C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\java\jinput\jinput-platform\2.0.5\jinput-platform-2.0.5-natives-windows.jar for job 'Version & Libraries': Local file matches local checksum, using that
[14:10:25 INFO]: Finished downloading C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\java\jutils\jutils\1.0.0\jutils-1.0.0.jar for job 'Version & Libraries': Local file matches local checksum, using that
[14:10:25 INFO]: Finished downloading C:\Users\Wes\AppData\Roaming\.minecraft\libraries\org\lwjgl\lwjgl\lwjgl\2.9.0\lwjgl-2.9.0.jar for job 'Version & Libraries': Local file matches local checksum, using that
[14:10:25 INFO]: Finished downloading C:\Users\Wes\AppData\Roaming\.minecraft\libraries\org\ow2\asm\asm-all\4.1\asm-all-4.1.jar for job 'Version & Libraries': Local file matches local checksum, using that
[14:10:25 INFO]: Finished downloading C:\Users\Wes\AppData\Roaming\.minecraft\libraries\org\lwjgl\lwjgl\lwjgl_util\2.9.0\lwjgl_util-2.9.0.jar for job 'Version & Libraries': Local file matches local checksum, using that
[14:10:25 INFO]: Finished downloading C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\sf\jopt-simple\jopt-simple\4.5\jopt-simple-4.5.jar for job 'Version & Libraries': Local file matches local checksum, using that
[14:10:25 INFO]: Finished downloading C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\java\jinput\jinput-platform\2.0.5\jinput-platform-2.0.5-natives-windows.jar for job 'Version & Libraries': Local file matches local checksum, using that
[14:10:25 INFO]: Job 'Version & Libraries' finished successfully (took 0:00:00.489)
[14:10:25 INFO]: Launching game
[14:10:25 INFO]: Looking for old natives & assets to clean up...
[14:10:25 INFO]: Unpacking natives to C:\Users\Wes\AppData\Roaming\.minecraft\versions\1.4.7-Forge6.6.2.534\1.4.7-Forge6.6.2.534-natives-27535127278243
[14:10:25 INFO]: Reconstructing virtual assets folder at C:\Users\Wes\AppData\Roaming\.minecraft\assets\virtual\legacy
[14:10:25 INFO]: Launching in D:\Dropbox\Games\Minecraft\My Profiles\Test
[14:10:25 INFO]: Half command: D:\Programs\Java x64\bin\javaw.exe -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -Dminecraft.applet.TargetDirectory=D:\Dropbox\Games\Minecraft\My Profiles\Test -Djava.library.path=C:\Users\Wes\AppData\Roaming\.minecraft\versions\1.4.7-Forge6.6.2.534\1.4.7-Forge6.6.2.534-natives-27535127278243 -cp C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\minecraft\launchwrapper\1.5\launchwrapper-1.5.jar;C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\sf\jopt-simple\jopt-simple\4.5\jopt-simple-4.5.jar;C:\Users\Wes\AppData\Roaming\.minecraft\libraries\org\ow2\asm\asm-all\4.1\asm-all-4.1.jar;C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\java\jinput\jinput\2.0.5\jinput-2.0.5.jar;C:\Users\Wes\AppData\Roaming\.minecraft\libraries\net\java\jutils\jutils\1.0.0\jutils-1.0.0.jar;C:\Users\Wes\AppData\Roaming\.minecraft\libraries\org\lwjgl\lwjgl\lwjgl\2.9.0\lwjgl-2.9.0.jar;C:\Users\Wes\AppData\Roaming\.minecraft\libraries\org\lwjgl\lwjgl\lwjgl_util\2.9.0\lwjgl_util-2.9.0.jar;C:\Users\Wes\AppData\Roaming\.minecraft\versions\1.4.7-Forge6.6.2.534\1.4.7-Forge6.6.2.534.jar net.minecraft.launchwrapper.Launch
[14:10:25 INFO]: Client> Error: Could not find or load main class Profiles\Test
[14:10:25 ERROR]: Game ended with bad state (exit code 1)
[14:10:25 INFO]: Deleting C:\Users\Wes\AppData\Roaming\.minecraft\versions\1.4.7-Forge6.6.2.534\1.4.7-Forge6.6.2.534-natives-27535127278243
[14:10:25 INFO]: Ignoring visibility rule and showing launcher due to a game crash
So "Could not find or load main class Profiles\Test". This profile is loading Forge 1.4.7. No-go, then?
If it's relevant, .minecraft is actually pointing to my Dropbox so I can sync between PCs. Symbolic links are the best.
Entirely understandable. I know I'm playing with unsupported stuff. Thanks for your help.