One bug report. With "transparent" maps with detailed preview maps such as http://www.calgarycachers.net/trailmaps/maps.htm the detailed maps do not show in Basecamp. I suspect it is a draw order issue with the preview map covering the detailed map.
This does work and has always worked in Mapsource.
I realize that you may just say that you do not support 3rd party maps, but overlay trail maps are very useful to many people and make more sense that integrating the trails into full map products.