pmendl -- I am facing a similar situation to you (at least it sounds like you are trying the same things I have been) so I'll be very interested to see if you can get this working. I'll share a couple things I've come across in case it helps. Launcher 2.x seems to be dead set on looking in the c:\%appdata%\roaming\.minecraft folder for a file called launcher_profiles.json file. (I am in a Windows environment but I think Linux is similar).
Using the --workingDir option only redirects some of the files, not all of them.
I was able to fake it out by relocating my c drive .minecraft folder to my d drive and I created a symlink.
In a command prompt I went to %appdata% and typed this (again windows cmd):
mklink /D .minecraft D:\Minecraft\.minecraft
That seems to force all the files to go into the alternate location. I can even get latest version of Minecraft and Forge to run. Problem comes when you try to load another version of Minecraft and Forge and point them to a different game directory. The launcher allows you to kick off another version and set the game directory. But when you go to install the matching version of Forge, it doesn't see the launcher_profiles.json file (even if you copy one in there) and throws the error: There is no minecraft launcher profile at this location, you need to run the launcher first!
With the old launcher is was very easy to have self contained "versions" that you could move around wherever you wanted but this launcher seems to tie you to 1 file the launcher_profiles.json. I have seen a lot of people refer to MultiMC for getting that flexibility so maybe something to check out (I haven't tried it).