I suppose the best option would be to try to build an actual hydraulic autopilot system, and install it with T-fittings. This would be the most reliable and user-friendly option, but also the most expensive if I need to buy a commercial hydraulic autopilot pump, not good if I want to do this on a budget. Is there any kind of "generic" 12v hydraulic pump that could work?
Another option could be to build a simple tiller autopilot, and make a manual bypass between the hydraulic lines. I'm not sure how much resistance it would encounter pushing hydraulic fluid through 20 meters of hose (I'd mount the bypass near the wheel if a manual valve) in addition to regular rudder forces, but it should be doable.
Edit: I guess that option wouldn't work, as I'd still need a way to disconnect the tiller autopilot.
Another option could be to build a simple tiller autopilot, and make a manual bypass between the hydraulic lines. I'm not sure how much resistance it would encounter pushing hydraulic fluid through 20 meters of hose (I'd mount the bypass near the wheel if a manual valve) in addition to regular rudder forces, but it should be doable.
Edit: I guess that option wouldn't work, as I'd still need a way to disconnect the tiller autopilot.