Use $2224 if your code is running on the main CPU, or $2225 if it's running on the SA-1.
Note that both of those registers are for mapping RAM, not ROM; see the "Super MMC" register info here
for ROM mapping registers, which is what it sounds like you're after if you want to expand the ROM.
The Super MMC registers are always written from the SNES side, but affect the address spaces of both CPUs. The lower 3 bits select a 1MB chunk of ROM to be mapped into one of the four mappable address ranges and the high bit enables/disables bank switching for that range (and basically just uses the default mapping if it's disabled).
I have no idea what the SA-1 support in Geiger's debugger is like (I suspect that address weirdness is its fault, not something the game's doing), other than that it doesn't support stepping through code running on the SA-1, which is a pain. I have a custom version of bsnes
that has more useful SA-1 debugging features; the support for it isn't as complete as I'd like, and I haven't been developing it much lately, but I can put up a current build if you think it would be useful