Hello benwiggy,
thanks so much for your reply 
!
Yes Add-on switches are nice but they only work when a CC is exclusively used for one particular purpose, which is not the case for the VSL.
I haven’t seen the Noteperformer Map so far, but ours already has 1500+ entries (not least because it already includes such “dummy definitions”). If we would really exhaust every possible combination (con sordino, sul ponticello, sul tasto, accents, map-specific playback modes, …) just to make this work, we are likely talking of 10000+ entries which is simply not feasible …
But this should ideally not be necessary: A way to specify that a given Playback Technique is simply neglected if the arising combination is not defined in the Expression Map (or even some kind of priority mechanism) would do the trick and make sure that every score is played back correctly with all the available samples and without a single superfluous entry in the Expression Map. Maybe this can be included in a future update of the Dorico playback.