If all Apple produces is a signed update (with logic that activates it only if it's the right IMEI) then it's not useful to anyone hacking any other phone. The

If all Apple produces is a signed update (with logic that activates it only if it's the right IMEI) then it's not useful to anyone hacking any other phone. The software can't be modified to work on them without Apple's cooperation, since any modification breaks the signature and you don't have Apple's signing keys.