A better way to prevent flooding is to ensure the return section of your sump cant flood your tank. Drilling a hole your adding points of failure: the seal around the hole drilled, the line can come loose from the power head, the power head can fail or just not come back on.
So measure the return section (in inches^3, gallons doesn't matter) then divide this by the length and width of the display and insure that when the return pump is running you still have that much space. But this will likely burn out your return pump if you loose siphon so you've got to choose which is less costly/annoying/likely to piss off the spouse: return pump or water damage to the floor/baseboards around the tank.
You'll also want to check the other way to make sure the sump can't flood with the return pump off.