You would need some sort of hardware serial number system for PCs and a whole security infrastructure to verify that the serial number being used for the unlock

You would need some sort of hardware serial number system for PCs and a whole security infrastructure to verify that the serial number being used for the unlock wasn't faked. Then you could do per-PC unlocks. I'm not aware of anything like that being widespread back in the Quake days, though these days every computing device you own probably has a unique ID burned into ROM somewhere.