I don't know if it already exists but I would like to know if it is possible to find an interactive map of Morrowind with Tamriel Rebuilt's Mainland included. Or is there a mean of including TR landmass on an existing map like http://mwmap.uesp.net/?

My guess is that these maps for the TES games were made from textured height maps. (just my guess on how it was done)
So the same process could be used for any mod content.

Regardless of how, the process should work for any esp/esm.