I was in Costa Rica when my navico autopilot died on me. I ended up
buying an autohelm from a guy who had exactly what you are talking
about. He had about a 48 foot heavy boat and he had a wind vane on the
back. The wind vane worked find in heavy wind but in calm wind or while
motoring through still air, he used the autohelm 2000 to move the trim
tab on his vane to steer the boat. It worked great for him and the only
reason he sold it the autohelm to me was that he was done cruising and
I begged him to death to sell it to me.

The way he had it rigged was with a bicycle cable attached to the end
of the tiller pilot and then the other end was attached to the rudder
tab. The ends of the cable were mounted and the wire was pulled in and
out with the pilot. Without drawing a picture it might be hard to
imagine but it was really a simple deal and as far as I remember, I
think he sailed the boat for several years with that setup and no
problems.