Just found out that the free Android GPS app OruxMaps supports .img files:
https://play.google.com/store/apps/details?id=com.orux.oruxmaps
Combine all the map tiles you want into a single .img file, then copy it into the mapfiles folder and select it as an offline map. Rendering isn't outstanding, but it does work.
Another use beyound the original idea of sharing mapsets for Garmin GPSr devices which map authors may have to be concerned about.
Does routing work?
Only mapset I looked at was Arizona topo. Didn't see any mention of routing, or any settings/options/buttons for routing.
This App works great on the type of maps that I make which are just maps of tracks and don't have a lot of other topo lines, etc.