I wonder if MMC5 audio has been implemented at all. If that's the case, MMC5's PRG mode 3 has the same capabilities of the Sunsoft 5B/5A/FME-7's PRG bankswitching. CHR Mode 3 is more or less the same as the Sunsoft scheme too. A mapper hack to MMC5 might be a good project...
EDIT: Never mind, MMC5 audio is not implemented either, though that is a little more understandable...