How would your MSU thingy allow more powerful echo?
...Using the spc700 for sound effects only?
Yes, you would have almost all of the 64K APURAM for sound effect samples alone. Thus you could have a much larger echo buffer.
There's a backup RAM register assigned to it too (BRAMR), with the description:
"Data becomes 'protected' when the BRAM flag is reset (0) after saving to the Back-up RAM".
Any time I enabled this emulation, various games broke. Some even turned on the write protection and then expected to be able to write to it. Made no sense to me. So no, that feature isn't emulated. Same thing goes for the SA-1 equivalent functionality. It's a shame too because it's a trivial one line of code.
Anyway, I don't recall the SuperFX bus layout entirely. I was under the impression there was only one RAM chip on SFX carts, though.