I've done It before. I drilled from the outside of the tank in and used tape, plastic bags to catch the shavings, water, and the glass circle. Just go take your time.
Pm me if you want more info.
+2. Without the proper amount of water or cutting fluid, the cut will get dry and the bit will get super hot. If you figure out a way to keep a steady flow of water aimed at the cut., I guess it could work. The thicker the glass the longer time the bit has to work.
If it's not near the wall and you have room to drill from the outside I'd do it that way. I wouldn't attempt it from the inside. My tank has a HOB overflow and if my tank wasn't too close to the wall I would have drilled it by now.