Nope - as I said in my question, these only work when creating new pages.
In fact, I think the easiest way to do this is an action-plugin - ideally it would use the COMMON_WIKIPAGE_SAVE-BEFORE hook, but that has a bug -
https://github.com/splitbrain/dokuwiki/pull/1696 - so it looks like an action-plugin using IO_WIKIPAGE_WRITE-BEFORE is the only solution. I haven't found one that does this yet - #surprised.