Unfortunately it doesn't...
Startup with no mods in the jars/mods folder works correctly, and my mod is reported as loaded.
However, putting the EBXL zip in that folder causes something to give up:
There are differences in the class it doesn't find, though.
For example, with the ExtraBiomes 2.2.2 zip, it can't find: forge/NetworkMod
With Thaumcraft 2.16C, it can't find forge/ISaveEventHandler
Oddly enough, the game seems to load fine with TFC's GemPack 2.9 (which is also a Forge mod that pretty much only adds ores and tools, its simplicity is why it works I suppose!
Scott: Thanks for the solution to my particular problem, although I'll leave my question up with its original intent (script? possibility of making one?) in case it's useful for anyone else...