They are not just covering their rears. The bottom is tempered, and will shatter if you attempt to drill it. My suggestion, don't do it.
If you need a drilled bottom, sell/return the tank, and order a tank with a non tempered bottom. It will take some time to get, since a special order has to be made. Also, it will cost a bit moew, for two reasons. (1) It's a special order. (2) The bottom will probably be a thicker glass to handle the pressure of the water column.
Another option is to drill the sides. The sides are not tempered, so you can make an overflow work that way...
Hope that helps.
Jason