I pulled the stud out of my floppy mirror by over-tightening the tension bolt but devised a fix.  I went to the local hardware store and got a small SS hex set screw and appropriately sized drill and tap.  I hammered the stud back into place (taking precautions to protect the finish and the threads by putting the nut over the threads) then drilled through the stud from the side of the base, tapped the hole and inserted my set screw through both base and stud.  The hex screw counter sunk into the hole which I filled with a dab of black silicon caulk.  Its been holding up fine, no one can notice the black mend on the finish and it can withstand as much torque as you need to keep your mirror from a floppin.

Good Luck
Ed in Syracuse
84' Westy
89' Wolfsy