Its been done before not sure where the post is. Holla Woody's Wheels he was involved. I think the R1ooR had an 18" front wheel so you can keep it tubeless if you want to stick with BMW rims. There have been plenty of 21" front conversions done. I have a R100GS front wheel on my 1100GS makes a big difference off road.
Link to a 1200 conversion interesting what is said about the 18' rear
And here is the 21" front conversion for the 1100 GS