No replies yet, I see. If you decide to tackle this, it might be a great tech article. Please consider taking lots of photos as you go.

FWIW, I have mixed thoughts on your plan. Using the main engine gives you the option for some serious watermaking capacity without worrying about large electrical loads. If you are also using a hi-output alternator to charge your batteries, you can run your main engine with some degree of load (the watermaker pump and alternator), keeping the engine happier and improving your overall efficiency somewhat. Many charter boats have engine-driven refrigeration and that serves a similar purpose. It introduces some challenges with component location and drive belts, I am sure, too.

OTOH, if your only option for making water (and charging batteries, for example) is your main engine, then you are SOL if that engine is out of commission for a while. Of course, it you use your watermaker to keep your 70+ gallons of tankage full, then you have a bit of time before running out of water is a big concern.

I have elected to go with two battery charging sources (solar and hi-output alternator) to enjoy the redundancy.

Good luck in your planning and execution.