I'm curious. What if you use the iPhone Configuration Utility to generate the output file? I've done this before to roll out multiple phone configurations that all had pre-configured email IMAP and SMTP settings for a large organization.
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.