Mapper 256 is used for OneBus multicarts. Because most of the games had to be modified to run on the OneBus hardware, the extracted games must retain the mapper 256 designation, as they still write to OneBus registers instead of whatever hardware they ran on when originally released as individual games. At least, that applies to normal NES games. As for Nice Code's games, since they were written for multicarts, you could say that mapper 256 is indeed what they usually run on.
I think I'm close to implementing the FC 600's sound hardware. I wasted time looking for a Sample Length register, when it's much more likely that the hardware just plays until it encounters an FF sample.