Just by comparison, Android supports on-device APN editing (see Settings -> More... [under Wireless and Networks] -> Mobile Networks -> Access Point Names).
T-Mobile actually encourages users to edit their own APNs for IPv6 support, and has run an IPv6 custom APN beta trial for a while:
I didn't alter any cell based network settings. Though I recall the output file from the iPhone Configuration Utility was merely a text file. I believe, though I'm not 100% certain, and this was around iOS3-4, that I opened the file in a text editor. I think I recall this because I came to the conclusion one could make a web based configuration utility that output the same data file.
I'm pretty sure it was just simple XML. It may be a binary plist type file now, but plutil should solve that problem.
Couldn't one manually edit the output file to contain the values they desire, import the file, pop in the SIM, and have a functioning iPhone on iOS6?
Just a thought based on some very old memories. Anyone care to fire up iPhone Configuration Utility and see what file type it generates. Even if not XML, a hex editor should allow the changes, no?. A small git-hub project could probably create all the needed profiles for the various carriers out there for unlocked phones.