Thanks again for your suggestion - I found a solution that works, based on your idea :-
1. Duplicate the FOLDER for the aircraft you want to use in multiplayer mode, and give the duplicate a different name (eg Cirrus SF-50 Red).
2. You will need n copies for n players (for example : Cirrus SF-50, CirrusSF-50 Red, Cirrus CF-50 Blue)
3. Do the same for each machine in the multi-player simulation
- so for 3 players, you need the same 3 copies of the aircraft folder on each machine.
4. When you run XP-11, you will now see n instances of the aircraft in the 'Aircraft' pane on each machine
5. On each machine - select a different aircraft and assign the livery you want to each individual plane
6. It works !
This solution isa bit clunky, but it does work.
I hope Laminar will try to fix this in a future release.