For some reason, I can't get the idea that pulling a trailer with a BMW could be a good thing for the final drive bearings. That side loading caused by the trailer in turns is going to be a tremendous strain on the final.
They're already wimpy enough as is and BMW isn't noted for a really stout final with all the drive failures of the R-series bikes.
Just seems like a really bad move, imo. I'd look for a bike with an axle and supported bearings on both sides before I'd use a BMW in that circumstance.