FOUND THE BUGGER.
There's a DC switch panel on the aft bulkhead in the engine room. Right next to the stairs and the generator. BEHIND that SCREWED DOWN faceplace are three fuses. The port-side one is for the windlass. It's a 150 amp ANL type fuse.
I only managed to find it by accident. We traced the 3/0 wire from the helm breaker back along the entire engine compartment. It comes down from the helm to the forward bulkhead, then across to port, then back along the port side to the aft bulkhead, then back toward starboard behind the panels and into that switch box. We lost the line when it turned back starboard at the aft bulkhead. I only managed to find it by accidentally resting my hand against that switchbox and one of the screws while also holding the button on my tone detector. Hearing the 'deedle/deedle' beep of the tone... EUREKA! It was my sweaty hand that made an antenna extension for the tone receiver to pick up.
It now sports a label to remind me. I do plan on figuring out what the other two fuses protect, but needed to wrap up for the day.
It would appear the breaker at the helm did not function. I've ordered a replacement.
If you're ever in a situation where you need to trace wires, a test tone probe is incredibly helpful. I can't recall what brand I have (it's at least 20 years old) but it looks a lot like this one:
Hope this saves someone else the time/trouble and hourly rate tech bill trying to find their Maxwell windlass fuse.