Perhaps you could trace the manufacturer through previous owners and send them an email with pics and ask if and how. If it turns out not to be tempered how will you cut a pipe hole, by etching a circle or some sort of abrasive cutter?
You can buy diamond hole saws that you run in an electric drill. I haven't done it myself but have watched videos of it being done and once the hole is started its just a matter of slow going until you cut through.