In the low winds that you are interested in you may find it works quite well without the control board. There is supposed to be some form of mppt but it may not be in the board. I think that is mainly concerned with noise reduction by braking above a certain speed.
I have never had one of those expensive toys to play with so I am not sure how easy ( or more likely difficult) it would be to incorporate 3 slip rings. Probably it is not worth the trouble. For the same volt drop, you will need 3 wires of a given size to carry the ac that could be carried with 2 at dc. My suggestion would be to keep the rectifier on the mill, keep 2 rings and transmit dc.
No doubt someone who has played with an AirX can advise you if they continued with the strange winding of the 403, I suspect they did and most of the claimed mppt bit comes from this odd winding. You may be advised to use the original rectifier or copy it I suspect it is a bit odd.
If you do get any high winds you are still left with the issue of control in high winds and the likely noise that comes with it.
Whatever you do it will still remain a poor machine for low wind areas.
Flux