A few months ago, I decided to test GBAHawk's multiplayer support using Game & Watch Gallery 4. After a few UI bugs were fixed, it seemed to function pretty well. I semi-optimized this TAS test, but there's probably a little bit more time save from better RNG in Donkey Kong 3 at the very least. This movie is missing the two Classic multiplayer modes, but those do also appear to function in the emulator.
It's very exciting to have multiplayer support so this game could finally be properly beaten. It's still a massive undertaking to optimize, but at least it's possible!
This movie was created in GBAHawk 2.1.1, but still syncs in 2.1.3, the most recent version at time of writing.