"How about installing an autopilot on a slow trawler with chain/cable steering?"
The simplest is to contact the fellow that was selling a Wood Freeman in the sales section.
The usual is to change the sprocket behind the wheel to a dual one and use a new chain on the new second sprocket.
If numerous long ocean passages are your deal, the WF would be worth the effort.
Personally I would not bother,, I would purchase an external drive unit as used on many wheel steering sail boats.
These use a large toothed pulley bolted in front of or behind the steering wheel.
A small motor operating a toothed belt makes the correction force.
These are modern sophisticated units , that do not require hyd pumps , rams or HD switching units.
They work because the wheel center does not shift over time , the joy of all mechanical steering.
You have it EASY!
Raymarine SmartPilot X-5 Wheel Pilot Corepack | E12133
Ray marine is only one example of this style autopilot, there are numerous others.
I would look on the SSCA forum for ideas from folks that have used them to cross many oceans.