you could do it fairly easy, just need one of these, and yours drill bits have to fit it.
http://www.leevalley.com/en/wood/pag...at=1,180,42316
most diamond hole saws have built in shafts though so might not work.
other option is to just drill around it, if you start on an angle and then shift to flat it shouldn't be to difficult, just use less pressure than normal and take your time. other wise get a new chunk of glass and redrill it, (this is on your overflow is it not) would be cheep for a small chunk of glass.
Steve
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