It would be very helpful if I could fuse/embed imagery into map tiles being served from online map services, e.g. Here, MapBox, OSM, etc...

For instance, imagine a map which has the height of every building in NYC added to the map. The building height data is not dynamic (let's hope!), so there's no advantage to draw the thousands of text values dynamically and basically cripple the map's performance. It would be much better to fuse those text blobs into the cached map tile itself.

Likewise, and more practically, any kind of aviation or marine chart is going to have tens or even hundreds of thousands of elements which are basically static (e.g. only change once ever decade or so)

Maybe this could be implemented as a special transparent layer which is a generated bitmap and is anchored to the map. Basically, a map on top of a map.

C.f.: FusionTables: https://developers.google.com/maps/articles/toomanymarkers