The Harley versions are prone to failure. I was advised to hold off until they get it right. I've been told the bars are hard to hang on to when the heat is stuck to FULL ON.
I wonder if the Beemer versions would fit?
AFAIK, the heated grips on the BMWs are a factory only option. Try calling up your local BMW shop and ask one of the techs there. They might be able to give you some pointers.