Assuming, say- 25 to 30mm tires, which fenders do you think would work best with a 650A wheel? One designed for 650B or 700c?

You can rework a metal 700c to 650b, so I'd imagine that you could rework a 650b to 650a

700's can be hard to get right. These Bluemel Populars look OK on an EA1 wheelset although they're still not dialed in. The mud flaps will start getting low on 650A's. Bluemel Lightweights are a better fit.

Thanks for the help, guys.

If I do this, my guess is it'll likely be with 650b mudguards.

I'm kinda considering the possible restoration of an older rando with geometry very similar to @clubman's EA1. Considering it: I've been offered the frame at a pretty fair price, but I'm working out a project-budget for it to see if it's really a do-able thing. I have two that I'm already working on, so it might be that I can sort-of dove-tail a third project into the effort.

That said, it might be a bike too far.

+1 on reworking 700c fenders. I used a lacrosse ball (hard rubber, about the size of a tennis ball) to widen (and reduce the radius of) my pair of VO hammered aluminium 45 fenders, rolling it back and forth on the inside of the fender for a few minutes, fit the 650b 38 tires perfectly.