I liked the idea of putting a mirror on the windscreen stanchion, so I had an old fender mirror laying around and thought that I would try to adapt it to my windscreen. I have a post 60k car with the screw on windscreen, so this method won't work so easy with a dzus type. I also used the wrong side (driver's) fender mirror for this, as it hangs down instead of pointing up, but it was all that I had, and I was just experimenting.
The threads for the windscreen mounting bolts are 5/16"-24, which also happens to be the same size threads as those on the fender mirror. Unfortunately, the stud is not long enough, and it is also part of the casting. I cut the stud off, drilled and tapped a hole and used a longer stud (about 2 1/2" in length). A couple of washers to fill the recess in the stanchion and it just screwed right on. I'm going to order another fender mirror for the right (passenger) side, and then it will point up. As it is, it just too low, but it does out work nicely.
I imagine that you can get a motorcycle mirror that will fit. You just need the correct threads that are long enough.