Another option is to get a hitch mounted carrier.
Bavautosports has a hitch for the E91 that they claim does not require any cutting to install.
I am an avid biker too. I have a road bike and a mountain bike, but I rarely use the mountain bike. I generally just fold the back seat and it fits nicely in my E91. However, I might be less enthusiastic about throwing a muddy mountain bike in there.
2009 E91 Montego Blue w/ Gray Dakota 6AT ZPP/Logic7/Sirius/Idrive/Butt warmers/6FL/PCD/CA/6NL/Xenons