Maybe add a slider to custom paint schemes that allows for transparent parts to be generated. This seems to me like a very simple solution to "glass," instead of actually adding a transparent part to the game. I say this because in the paint tool you can change the glossiness, or how reflective, parts are. This and you can give things tints, so you can make "stained glass" in the game, again, without actually adding a new part.
This has been added to the game with the latest update. No, this particular suggestion was not added to the roadmap, but the devs added a transparency slider.