The technical aspects start going over my head very quickly, but I remember from my days with an SNES copier talking on relevant forums that the SFX, the SA-1, the SDD-1/SPC-7110 and maybe one or two others aren't easily put into a flashcart or a copier because these chips are given customized access to specific parts of the ROM that varies case by case. They also have their own work-RAM (~32kb, I think). It's probably not altogether impossible, but I wouldn't be surprised if it wasn't feasible with the current methods.
The following chips are supported in the SD2SNES:
BS-X memory map / Satellaview base unit registers (clock)
DSP1 / 1b
DSP2
DSP3
DSP4
ST-010
Cx4
MSU1
S-RTC