That would be far too provable and suspicious, since it'd have to be done 24/7 and would rule out the heisenbug route.

If you were to do such a thing, you'd do it by making the machine overclock itself too much occasionally (after the 'artificial use-by date' has passed, thereby incurring physical damage at random intervals. Although really, it would be easy to do without blob, too, if you're doing the design of the physical chip.