Anyway, if you're good with electronics (as I assume you are, since you want to build a back up unit that supports SA-1 and DSP) wouldn't it be better if you bought a SWCDX2 or any other back up unit, and experimented some with it, and maybe you could find a way to play SA-1 games and SFX2 and S-DD1 games on a copier with a dummy cart or something...
mikael