I have done some research over the years on Tempered Glass for a couple of reasons,
1, I do glass engraving and sandblasting, usually done on show cars. It's not popular now, but in the 80's I did tons of engraving during the "Van Craze"
2, I have chopped a "65 Chevy Van 10.5" and currently have an '83 S-10 that is I chopped 4.5"
I once did a bay window on a van and it exploded a week later, upon doing research I found out altering the glass by cutting into it can cause a failure at anytime. It was boiled down to it probably exploding due to expansion as the van was parked in the sun when it happened.
When I choppoed the S-10 I now had a customer who owns a fairly large glass shop, he explained to me that things such as drilling into tempered is not done. As posted above, it is drilled then tempered.
The moral of the story, unless you know for sure it is NOT tempered, leave it alone. You could successfully drill a hole in tempered, only to have it explode later for no reason.