I had one of my more inspired brainwaves 8) today whilst using Mr. C. Davy’s Most Excellent Layout Designer for the first time.
The Browser can already list an .mlp file (Playlist) in its own Browser pane.
If it could also list an .mlc (Cart Set) file from the Browser Add menu, this would instantly solve the ‘copy/paste carts from one Cart Set to another’ problem, because the Cartwall could be used as a ‘scratchpad’ on to which you could drag/drop existing Carts direct from a Browser Cart Set pane.
It would also provide similar functionality to the JazlerShow! Collect function, and if you add a small separate Playlist for the specific purpose, voila! you have the equivalent of the JazlerShow! ‘stacked’ cart player. 8) 8) 8)
(I told you this was a good one!) ;D
Have I convinced you yet, Torben? 
Hopefully not too much work to be worth implementing this?
BFN
CAD