You looking for a dedicated winter tire set-up, too? That's what I'm scouting around for.
It looks like two wheels w/tires mounted might fit in the trunk...but I'm not sure if 20" wheels w/tires mounted will fit in the rear seats (or go through the rear doors). These f06s just don't have the back room space of the LWB e38 (LOL).
Hmm, I wonder if it will work if you lower the rear seat backs...you could possibly push at least three mounted 20" wheels in...then wrangle the 4th wheel on top of the others.
Hopefully someone that has done this will chime in and give us hope. Come'on people...let's keep hope alive. Help us f06 noobs with this logistical question.