Robin, I've tried this:
Seems ok from a dry run without fluid or servo. Full test will have to wait until I've put fluid back in the system. Going to need to glue this in place some how, probably with silicone sealer. Given the shroud (?) is so big on the BMW master cylinder I can see this ever falling out, but it could reposition depending on the speed at white the piston retracts when you lift your foot off the brake.