I added one this way: node/5163082067. Here I used note=prayer room, but perhaps it is better to use description tag instead. You can use the level key for objects in different building levels.

Very nice! I also like OSM History View by Pewu but it seems to struggle to visualise ways that have too many nodes or versions, I think. Anyways, yours is definitely another tool to add to my OSM bookmarks. Cheers!