Your better option is just to purchase a non-branded (unlocked) phone to begin with.  As you are correct - with a recently released device, you are paying the vast majority of the purchase price when you purchase it with prepaid service anyway. 

There are quite a few disheartened at&t customers who have purchased recent models from at&t (Garmin G60, SE X10a, etc. at at&t's full retail, no commitment pricing), yet at&t refuses to provide the subsidy unlock code.  There was a fairly recent ruling (state, I believe) that mandated the subsidy unlock codes are to be provided even for devices with an exclusive sales agreement once the age of that sales agreement has moved beyond 10 months.  But at least to my knowledge, at&t still "hides" behind item #4 as stated in the prior post, while of course the manufacturer states they can't provide the code because at&t has to provide the authorization.  IOW, the typical finger pointing designed to avoid accountability for the device vs. the service that goes along with the subsidized equipment purchase model.