I think you can do it.
You can view your transparent mapset (.img file) on top of a solid map (road or topo) using Mapsource or Basecamp.

You'll need to install [Only registered and activated users can see links. ] and [Only registered and activated users can see links. ]

You'll drag 'n drop the two .img files (transparent and solid) on Gmaptool and JOIN them as a new mapset using solid map's MapID for the new mapset.
You'll drag 'n drop the new produced .img file (your new mapset) on Gmaptool again and now you are going to SPLIT it using "files for Mapsource" option and you'll use the MapID that you've used above (solid map's MapID).
Run install.bat in the folder with the splitted image and you're ready!
You must not have previously installed the same solid road or topo map in Mapsource (you must not have installed at the same time, the solid map standalone and in the new mapset because they have the same MapID).

Below there is an example I've made using the solid Greek road map Autodrive and the transparent map of Anavasis (that has only contour lines, rivers, streams and places)
(You can see the river double, as each of the river's blue line is from each map).

con01



Another example is again the solid map Autodrive and a custom transparent map with only contour lines for Greece.

con02