I'd like to place a tile service of a georeferenced map from DavidRumsey.com in a CartoDB map, but I don't want it to be the 'basemap'. I want it to lay on TOP of the basemap, which would be any of the default choices.
Any way to do this within the GUI? Obviously it can be done with Leaflet, etc... but I'm writing guidance for a non technical class to use CartoDB for a History of San Francisco project, so I'd like to keep them in the GUI.