I don't know of any insurance companies who will allow you to do a temporary transfer onto a car you own yourself. And a back-forth permanent transfer will get expensive. You might be better off going through Chubb or something and get fleet insurance.
The shinners do have the ability to surprise me, but I'm not sure that someone struggling to insure his fleet of BMW's is the sort of cause they'll take up. Not unless theres a lot of votes in it of course.