Thank you for the replies, there's some very useful ideas there.
Supportive / stiff non-climbing shoes, good idea, some of mine are pretty soft. I can get stiffer shoes or rigid insoles (or construct a stiffener).
Pete: Yes it's on the bottom, it could be what you suggested, I think it's some sort of strain at the joint.
Chris: Given the general functionality I am sure it is just a strain, it's quite specific what makes it tender, and there's little / no pain away from that. Edit: I'm going to Pfalz, not Pedriza thank fuck. I was looking through guidebook photos yesterday and I think that Pfalz is pretty generous with features and friction that should allow working around the toe more than many other venues.
Climbing-wise I went to the Boardroom last night, with a clear plan:
1. Tape up well to see how supportive that was.
2. Break in my downturned shoes and see how supportive they are.
3. Work down from positions that feel completely fine (outside edging, ball of foot) to see when it starts to feel risky, to know what restrictions I have.
4. Try to find a way to make the god-awful beginner style "whole side of foot on hold" clomping feel less incompetent and inelegant.
5. Be extra careful to avoid any discomfort let alone re-aggravation.
The results were:
1. I improved the taping with more support / restriction to stop the toe flexing backwards, and this lasted through the session and kept it supported.
2. After softening my shoes earlier I could wear them fine, and again they supported the toe and stopped it flexing back.
3. This was fine, I found I could stand on some smaller holds as long as I was careful and kept the toe pointing more down than flexing back. I didn't try any positions that could flex it back.
4. No comment.
5. Yes I did this. Every single right foothold that wasn't a proper bucket, I looked down, assessed how to use it, and if it wasn't strictly outside edging / ball of foot / heel-hook, then I tested it gently before weighting it. Quite weird climbing like this but I didn't get any discomfort through the session. I do have a bit of surface tenderness this morning (different to the strain tenderness) which might be a combination of the taping and old scar tissue from when I gashed the underside on a hidden sharp stake stub whilst walking around the soft comfy grass above Craig Dorys in 2007.
Obviously I am not going to push it any further now. My plan is to stick to what I intended in my first post but with the addition of stiffeners / insoles.