I've just been testing with .BIG, and good news:
Generals.exe -mod Something.big
works if the .BIG file is in the "My Documents\Command and Conquer Generals Data", but not just that, there's also another way which the game accepts:
Generals.exe -mod "(Program Files)\Command And Conquer Generals\Mod0001\Something.BIG"
How much of the path should be included I haven't checked yet, but since this works, it's good enough for me

Single-file mods, here we come!!!
I have an idea for a Generals Mod Selector, where you can select which mod you'd like to run (and the mod is in a single .BIG file in the \Mods subfolder in your Generals folder (in the Program Files)). The program would automatically detect any big files in this folder, and for example would extract something like "modinfo.txt" from the .BIG file to get the mod name and an icon. How does that sound?