I love the story thank you! Zero off is tightly integrated with the engine ECU, I'd be super interested if you could reverse engineer it and make the head unit work with other ECUs but that seems unlikely.
In addition to that, the zero off pull is dependent on the engine, prop and zero off calibration i.e. how zero off has been custom calibrated for all the variables. For example, i hear that certain boat/engine combinations suck with zero off even though that motor has plenty of power. So even if you could integrate with zero off, you still need a calibration that works for your motor/prop/hull combination.
Perfect Pass works with many digital throttles. It works with Yamaha outboards plus a number of inboard engine brands. I'd be talking to them to find out what standard you need to communicate with. If you want to run your own EFI system in an old boat, then PP is going to match that well. It won't be _exactly_ the same as a new boat, but then it was never going to be anyway.