Just re-read this - you mean multiple GROM bases on GROMs 0,1,2? Pretty sure I added that for you.
Maybe I'm confusing intent, but if that's what it was, you set Emulation/bankedConsoleGROMs in the Classic99.ini to 1, then load as normal with G1, G2, etc.
That's a hardware mod on a real console, so it doesn't make sense to do it per cartridge.