I can't seem to find anything related to doing it. If it's possible, I'd like to look into writing a rom/save dumper that uses a GBA and a GBA/GCN adapter.
It'd send a rom to the gba to boot, the gba would then read the rom or save from the cartridge, then send the information back to the gcn via the joybus, and the gcn would save it to an sdgecko.
So it'd involve writing both the gcn application and the gba application.
Can I use libogc to communicate with a GBA via Joybus?
Re: Can I use libogc to communicate with a GBA via Joybus?
There's nothing in libOGC stopping this other than the fact that no one has really documented this protocol. You'd need to figure it out by reversing a game/piece of code that already does this, better yet if one of those games has a full ELF file or .map with symbols left on the disc 
Re: Can I use libogc to communicate with a GBA via Joybus?
It's documented in Dolphin's source code. Pretty simple stuff, all done through the global SI command buffer. The first command specifies if it's a READ (expect a WRITE command and 4 bytes in response) or a WRITE (followed by 4 bytes to send, expect a READ response).
