It seems that Mapinstall has integrated similar protection mechanism, as Mapsource - the problem was solved using patched Mapsource (in windows).
You can try the following (requires windows ). Procedure is described here: [Only registered and activated users can see links. ].

If you have parallels, you can do it directly in folder where gmap is (home/library/application support/garmin/maps) - in windows envirionment extract files within the .gmap folder and run un.bat. It should result in a completely unlocked maps.

If you don't have parallels, do the same procedure on windows computer and after having unlocked the map, transfer it to your mac, delete old gmap containing locked map and repeat the instalation. Restart RoadTrip and Mapinstall.

Feedback would be appreciated-