I would say wait for T77269: Make it easy to modify which thumbnails are included in the viewer and the wontfix it. Full-screen overlay maps would not be terribly interesting, even if they were displayed properly; displaying them properly would mean showing two images on top of each other, which is a lot of work to do in a robust way (it took a long time to make the resizing, switching, preloading etc. of the image bug-free, now all that would have to be done again), and the whole thing would be very enwiki-specific, which is generally something to avoid.

If card #294 gets implemented (which is straightforward), site admins can just blacklist the overlay maps so that the lightbox does not open when clicking on them. That's a bit inconsistent UX-wise, but probably unavoidable, there are even more evil images (like the maps which display text and markers via absolute CSS positioning) which we definitely want to blacklist.

Mass-removing the Multimedia tag from MediaViewer tasks, as this is now being worked on by the Reading department, not Editing's Multimedia team.

Templates generating such things, can output the class noviewer, to avoid a confusing situation.

For anyone interested in an on-wiki solution, huwiki has a gadget (JS, CSS) that enlarges location maps as expected. It has quite some burnt-in Hungarian strings, which need localization, but it can be used as a starting point.