I think you understand the problem. Wind
vanes can not use the hydraulic streeting system to turn the wheel because of the slip inherent in a hydraulic system.
I think there are only four alternatives:
1. An auxillary rudder style. (e.g. Hydrovane)
2. Turning the rudder directly, i.e. the Sayes Rig.
3. A custom "trim-tab" style addition to you rudder. Depending on the type of rudder and the level of mechanical expertise available this might be impractical.
4. Connecting any other type of servo-pendulem windvane
using the emergency
tiller or other connection directly to the rudder post or quadrant. This is likely to be the best, but most expensive and potentially disruptive arrangement.
Not having any direct experience with this kind of interface, I'll leave it to others to give you the pros and cons of the various systems. I'd strongly suggest that you talk with someone who has REAL experience with windvanes on you particular boat, or at least one of a similar size and rig under the conditions you plan to use it. What might work in light winds might not in heavy weather
and vis versa.
You have a large and heavy boat. Be sure that the advice you are getting is appropriate. Maybe the manufactureer can help with suggestions or put you in touch with other owners?
You might also consider buying
the book The Windvane Sef-Steering Handbook
by Bill Morris.