Trailer hitches for the 2 series including the M235i are available in Germany and most of Europe. The problem is that BMW has approved the use of the trailer for all 2 series coupes but not the M235i. It would be illegal to use in Germany because of stringent technical inspections. We don't have those in the US.
Link in German for M235i hitch:
http://www.kupplung.de/westfalia-anh...5-11599-1.html
You can find the same site in English through:
http://www.rameder.eu/
The exception for the M235i has probably to do more with engineering concerns from BMW. Probably overheating when you carry heavy loads and other issues with high performance engines. That's probably why none of the M cars are approved by BMW for trailer use.
I checked rear space under the M235i and 225i and I don't see a reason why the hitch wouldn't fit. I plan to get the Oris version later in Spring when I visit Germany. Will let you know how it works out.