So I've had to deal with the glass problem myself now. The attached picture shows the issue. A curved windshield.
The initial plan was to have the exterior surface of the windshield set to MSFS_Windshield and double sided so you can see rain drops from both sides. The interior surface would be set to MSFS_Glass to get nice reflections of the interior. Having both materials visible inside the cockpit would provide some depth to the windshield (you can see the inside and outside surfaces of glass).
I am fairly convinced that using double sided material is a big problem for curved surfaces like this as it caused no end of graphics glitches where the back surface was drawing in front of the front surface.
In the end, after much tinkering, I came up with this solution. The exterior surface points outwards and is single sided only and set the MSFS_Windshield. This surface was then duplicated and the normals flipped so they point inwards. This allows the rain drops to be seen from the inside. Finally, the inside surface of the windshield is set the MSFS_Glass to provide the reflections of the cockpit interior.
No double sided materials were used.
As a final step I named the parts in an order that puts the exterior surface pointing inwards first (Cockpit_window03), interior surface second (Cockpit_window04) and exterior surface pointing outwards last (Cockpit_window05). I have no idea if putting parts in alphabetical order affects the draw order but I did it anyway. Note that the 3 textures used for the windshield all have Draw order 0.
As far as I can tell this seems to work for my aircraft.
Final point, If using the Blender exporter check to make sure that your export options have Include - Custom Properties ticked to make sure it exports the MSFS materials code. At some point I had this unticked and I'm not sure what effect that had.
Very final point. I'm not sure if this will be the final format I use. While the cockpit reflections of the glass material are nice I am finding the material itself somewhat difficult to work with as it creates a very dark texture plus there are numerous artifacts in the reflections that aren't pretty. Changing the direction of the eyepoint also tends to affect the reflections. Something for me to worry about in the future.