I can't imagine how would shadow calculations for cockpit objects impact the performance if done on the external, technically it can work in a way to put all objects that are not controlled by cockpit.ini with the external mesh and the just make gauges, lights, joystick, mirrors, hud etc in the separate cockpit lod, so yes it's definitely doable however you couldn't switch it off for cockpit only, it would be either having shadows at a very likely huge performance impact or not having shadows at all.
Sophocles if you really do decide to go that route I too hope you will make two versions or atleast test it on variety of systems before release so you can see true performance impact...