Cool idea, but it's a ways off before I do this.

If this is of any help: https://github.com/FIX94/gc-gba-link-cable-demo

I saw this, it looks tremendously useful.

any updates on this?

Is this closable as Dolphin introduced in-Dolphin support for (clock-matched) ability?

This actually refers to using a real GBA to control the Wii port of mGBA, not a GBA-GC interaction like Dolphin has (and mGBA has had for a few months before that too).

I completely missed that, apologies!