There seems to be a bug in the current version of Basecamp that can cause the basemap to be displayed at the same time as other maps. In the course of discussing this, one of the Basecamp developers says that adding support for transparent overlays (such as MyTrails) is actually on their "to do list". :)
See: https://forums.garmin.com/showthread.php?32540-Bug-BaseCamp-shows-roads-twice&p=155857#post155857
Falagar's got a lot of stuff on his todo list. Middle mouse button pan is another request that hasn't made it into this latest version.
True. However this is the first time I've seen any acknowlegement that they are considering adding this feature.
Actually, BaseCamp has been able to show 2 maps at once for some time. I noticed maybe a year or so ago having a single time transparent map on my datacard, BaseCamp read the transparent map from the card and overlayed that onto the BaseMap.
So I wouldn't get my hopes up that something is in progress, I think the issue is just coming into light.