Would it be possible to add BS-X dumping (but definitely not writing!) to the Retrode with this information?
http://wiki.superfamicom.org/snes/show/BS-X+MMIO
EDIT: Maybe move this to general discussion?
I have a theory on how to dump the BS-X carts.
Perhaps put the BS-X adapter in the SNES slot with a Satellaview cart in that, and then compare the ROM of that to a ROM of just the adapter. Delete the code that's the same, and there you go.
If you want to play a Satellaview game, simply add the Satellaview game's code back into the adapter ROM and play.