I took the Super Game Boy code I made for Petal Crash GBC and made a code example that shows off booting custom SNES code and then having the Game Boy side send files to the SNES as needed.
Code and ROM (under "releases"): https://code.novasquirrel.com/Maffi/SuperGameBoyStorageServer
Art: https://opengameart.org/content/tiny-16-basic
The text at the top is the result of asking the Game Boy to calculate $AA + $BB and send the answer to the SNES, as an example of doing other tasks than just getting files.

