Actually, I believe a solution at least markedly similar to Activation Lock is REQUIRED by law in at least CaliforniaBS. That method to iCloud unlock is for the less than 1% since it requires expensive specialized equipment and skills to accomplish. The only reason iCloud lock exists is Apple greed to limit used equipment market.
http://www.techrepublic.com/article...i-theft-law-iphone-appears-to-already-comply/
I do agree that Apple should digitally sign their Serial #'s.
Just encrypt it on the device, as long as the key is not compromised then only apple could ever decrypt it.
The issue is HOW to encrypt them; especially ones already out in the wild; any software update that would encrypt the Serial# could be decompiled to expose any encryption key.
But for new devices, they could be encrypted at the factory; Apple could provide their manufacturers with a list of already encrypted Serial #'s