what brand tank is the 90g, you can generally find out if the glass is tempered on the manufacturer's site or through a google search, but you can also check with a smart phone and some polarized glasses I believe it is.
Drilling it will be way more fool proof then a hang on the back overflow and would be my preferred method, and as for installing baffles in the sump that's the easy part, just some aquarium safe silicone and get a glass shop to cut a few baffles and glue them in, nothing to drill down there.
I'm down in your home area almost every day for work, so if you need help with something early in the afternoon I can generally lend a hand.