Comment by mkl
These replies prompted me to go looking. Firefox Android (which I'm using) does have an option in the settings under Accessibility called "Zoom on all websites". It works!
Chrome has a similar option, which also works on this site. I expect this might break a few pages, but Google Maps and OpenStreetMap work fine, with pinch zoom zooming the map when you do it on the map.
Awesome. Somehow I've missed them introducing that, as a long-time Firefox Android user.
And frankly it's in the wrong place if you ask me.