Another option would be to get a HOB overflow, but I can't make a recommendation because I don't use one.
Tank sides can be drilled, there's some threads on the DIY forum about it. I believe it requires a particular type of drill bit. One problem is that the holes need to be fairly large, over 1" IIRC. I don't think bulkhead holes on the side near the bottom of the tank would be drastically different than holes in the bottom, maybe the flow might be reduced a bit.
You could also ask the LFS if they know any glass professionals that would drill it, or look in the phone book and call glass shops. Maybe you'll get lucky and find one that's done it before.
What ever you do if you go with drilling, I'd get ahold of the bulkheads first so you know exactly how large the holes need to be.
Of course, you'd then have to get a hold of the overflow itself and install it. I don't know if AGA sells them seperately.