I be in the US in a few days so will be able to give this a go and let you know results.
From what I have gathered combined with assumption/educated guess:
AT&T obviously will have all of the IMEI numbers for the phones they stock/have stocked.
They could (if it's possible) also have the IMEI's of other providers GSM stock in the US.
But I highly doubt they would have IMEI's of handsets that they do not/have not stocked, that would take A LOT of work.
During the online registration of GoPhone SIM cards, AT&T asks for the IMEI of the phone you want to use. Apparently you can just type 111111111111111 (1 x 15) and it will accept it.
Again all just theory, but I think the general consensus is that if it's not a phone that AT&T sell/sold, meaning they don't have the IMEI, that you will get away with it.
we know all the phones we shipped and all the connections from companies using our service,
we have access for tech support and savedesk (customer svc), but general sales support does not have such tools to check such databases