I've just been looking at some already-dumped multicart ROMs, and it appears the majority of multicarts with a small number of actual games use mapper 8, which is fairly easy to implement. I also intend to dump the games on my MiWi2 cartridges, which are OneBus so should pose no problems. It'll be some time before any actual dumps probably appear though (I've got to buy stuff, hack stuff, solder loads of wires, write the code etc.) I am also designing a OneBus dev-cart, and combined with a free C compiler available from VRT, the possibilities could be endless.