I put a Trijicon RMR on a Glock 23c I bought at an auction and love it. I was having difficulties with cataracts and it really helped. Once I had surgery and have clear eyesight in my right eye, it's even better. I have owned a 6906 for a long time and ever since I put the RMR on the G23, I have wondered if there was enough unobstructed metal on the 6906 slide to mill it for an RMR. I did a lot of searching, and haven't found anyone advertising doing that operation. I assumed that if it was possible, someone would have it listed as an operation they would do.
The alternative would be a laser, but I've not found one that is frame-mounted other than one the clamps onto the trigger guard that I don't want to deal with. The 3rd gens have been out of vogue long enough that I guess most of the laser folks have ignored them. I even thought about trying to drill and tap holes in the dust cover of the 6906 and screwing on a rail so I could mount a laser. But every time I think about it for a while, I decide to leave it alone.