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.

Opera Mobile has a force-allow-zoom option

If only Opera was still Norwegian...