I think you may confusing the hosting of midi effect plug-ins. In AUv2 the AU does not require any audio buses (which you can check with auval). However, when hosting a midi effect auv2 plug-in you must call the
AURender functions as if there was a single output audio bus with a 0 channel layout, so that the plug-in can receive timing information. But again, this is only from a hosting side.
As I said, I can certainly bodge the AUv3 code to always have an audio output bus, but I don’t see a way around point 2 in my post above - i.e. needing to claim that the AUv3 is a synth. This would mean that the AUv3 would also appear as an instrument in GarageBand which would be a little confusing to the user.