Believe me, if you are man enough to replace the quarter, you are certainly man enough to remove and install the window!

imp:
You are GOING to have to remove it anyway, there IS rust there. Every single GM car of that vintage has "some" rust there. The ONLY way to fix it properly, is to remove the window.
The rear window is tempered glass, it is very tough, unlike the windshield which easily breaks. A cut out knife and a little time and it is good as out. The piano wire is a possiblility as well.
If you want to you can cut the window out, and while you are not working on the car, just set the window in and duct tape around it. Then you can easily pull it out to work.