I met with two Apple Genius Bar staffers today, and neither knew the answer. This is what they do know:
1. Only ATT&T does US post-contract carrier unlock. Verizon/Sprint can't/don't. So nice advantage AT&T.
2. If you buy an unlocked phoned it will show as unlocked in Apple's database and if you have it serviced it will be replaced with an unlocked phone. However, if you have a post-contract phone unlocked they don't THINK Apple updates the database used to check replacement status.
3. If you are post-2 years with AT&T, but haven't switched to a new phone/contract (so old contract is current) there is an option. What happens is that in this situation you do the glass repair / refurb switch, then put your SIM into your phone and AT&T changes the IMEI associated with your current (completed) contract. Then you go through the unlock process with AT&T.
They've never come across my situation. My guess is that there's no way to do it at this time.