launching other SITL models

I have been trying to help a student trying to do some work on the quadplane tilt-rotor support. He was trying to use MissionPlanner and found it impossible to get the quadplane-tilt model working within the Simulation launch in MissionPlanner. Using --model=quadplane-tilt changes the physics, but MissionPlanner kept forcing Q_TILT_ENABLE=0. Even editing sitl/default_params/quadplane.parm didn't work, that file got overridden with the old values on every launch.
We need a way to be able to launch any SITL model
For this student I've pointed him at WSL and sim_vehicle which will get him going, but I'm creating this issue to note the problem

Start with MissionPlanner's Simulation launch handling and trace how the --model=quadplane-tilt argument and sitl/default_params/quadplane.parm are processed. Done means Mission Planner can launch an arbitrary SITL model, including quadplane-tilt, without forcing Q_TILT_ENABLE=0 or overwriting the model's parameters.

Written by the indexing model from the issue text.