You've told him that you Overclocked the chip, which voids the warranty. He could add it to a "Void database", and he'd be completely within his rights, you wouldn't have a leg to stand on.
Still there are things you can do to their CPU that WOULD void the warranty. Such as LAPPING them and taking their protective coating off as I have done with mine.
I think so long as only the batch number is provided they aren't going to put anybody in a list of voided ownership. I think that someone is reasonably impressed that the CPU has gone past their list specs and would honestly like to know which batch so they can pull one of them from the vault to see if they can see why it hit those numbers. Sort of a test.
Give him the batch # please.