Thanks for your detailed description, Dewdman42, and I endorse your suggestion to VSL. It may be that I've misunderstood something as my procedure seems simpler than yours. My task was initally simply the following -- to merge the Special Edition mute trumpets and trombones contents into the primary instruments. I just did the following following Andi's suggestions:
1. go to the mute instrument and right click in empty space and "copy root slot"
2. in the main instrument, click on the + to create a new articulation in the articulation column and call it "mute"
3. right click and "paste slot". The entire mute tree structure is then copied into this slot. In the Dorico Expression Map, you simply add F#1 (assuming default mapping) to access the mute articulations you want to use.
I can understand that mixer impulses are different between different instruments and that they wouldn't be carried across but I can't see any mixer settings which differ between mute and standard trumpets. In practical terms what, if anything, is it I'm actually losing with this approach?
David