You can do it...
I have a link for you...
and I too have had the same trouble.
I used some little diamond bits to drill through the glass, then like a puzzle where you shake and rattle the little ball into a hole, got the screw back into the threaded area and screwed it back down. On my 650 I've had them wiggle then the screw come all the way out. I've had both screws do it...don't let it get that far. In all I've had 4-5 speedos on my 650 over more than 100,000 miles. They vibrate, the screws pop out, the needle flies off, the cable attachment busts loose from the plastic housing...ect, ect.
My friend carfully pried his bezel apart and got it back together nicely. And here's a simular method;
You see that I had already generated a lot of dust with the wiggly one I repaired, but it serves as a back-up now.
I have actualy turned over some of these speedos. all on the same 650!
Funny to see it go back to zero again!