I don't know about the SFX carts but for the SA1 carts it may be possible to modify the backup unit to allow the cart to be played through it by disabling the backup unit's lockout chip and routing the lockout signals through the cart. This is good for somebody who wants to try dumping data out of the cart. It sounds like your goal is to be able to play many SA1 ROM images on a backup unit by using one "key" cart with an SA1 in it. This won't work because the program for the SA1 cpu has to come from the ROM on the cart itself which means that for every SA1 game you want to play you need an SA1 cart with that ROM on it.
Cheers