I took the plunge and bought the 17" space saver spare tire kit from European BMW, and tried it on over the weekend.
I can confirm that the spare tire (17x3.5 ET8 with 125/80R17 tire) from European BMW fits on a US-spec M235i with no clearance issues. Being an OEM solution, this does not require a hub centric ring.
Part #36110445045 (wheel+tire only) or 36112159860 (Kit)
http://www.realoem.com/bmw/showparts...=36&fg=95&hl=1
Pictures to follow.