Hello,
sorry for the necro-bump.
After doing the mod I was testing saturnu's codes for Mario Tennis, and found an issue: writes to the GB cartridge would fail.
Well, here's my revised version (PAL):
81122228 0000
8112222A 0000
81122410 0000
81122412 0000
Done the hard way, by NOOPing BNEZ instructions until I found the correct one.

Tested: character unlocks works both way, EXP does transfer & n64 data is shown in GB mode.
Off topic:
I had issues with Stadium 2 while using a patched crystal rom. (the "at startup ask for clock setup" one, useful as the original everdrive GB does not have a RTC clock) It would read / write saves correctly, but the game boy tower would throw an error. Well, I double checked the GB header - there's another checksum, the "global" one. It is not checked by super / game boys. Useless? GB tower does check it! So please fix it if you care about GB tower + a patched rom.