For a wind vane autopilot I think we need to consider two scenarios:

1) a wind shift with no increase in wind speed - means no problem except that we would need an alarm for course change

2) a wind shift with a more or less violent increase in wind speed - lets say a micro burst at night
In this case the speed of the proa will also more or less violently increase as the apparent wind moves forward with the wind vane compensating for that by bearing away to keep the previous angle of attack. If that happens too fast there is some likelihood that we will stuff the lee hull into the next wave as the wind force on the sail is also pressing the bow down initially. Of course a wave piercing bow would lessen that problem of violent acceleration und subsequent stuffing to some extent. Anyway - a dampening factor for the wind vane to allow the sail to be partly stalled a little bit longer could be in order to allow the boat some more time to settle into the new speed regime.

Could that be done with something towed - like for example some sort of a home build propeller driven generator? Speed increase would mean more drag on the generator to more dampen the wind vane. Also the increase in electric current could be used as an alarm for speed increase and associated course change.
What did I miss?
Heinrich