Better: an NES mapper and a full-on Chinese Starfox ripoff in the style of those (arguably amazing) Donkey Kong Country ripoffs.
My god, that would actually be doable. The SuperFX normally has access to the SNES's bus, but there's nothing to say you couldn't just wire its bus up to a CPLD, which then arbitrates between it and a couple of roms, between it and some RAM, and between the 6502 and some RAM.
The only issue I can think of is that tilemap indices on the NES can only be from 00 to FF, but a full 256x240 buffer would require a full 10 bits, from 000 to 3FF.
