I would start with Compass Lake Engineering in Florida. I am not telling you he will do it, but in my opinion, Frank is the master of AR barrels. He does some custom chambered .308AR barrels for us, but then again, it is not a one-off. Frank has done alot of research into what works and what does not. The only reservation he might have -- well, maybe two reservations -- is 1) will the fucker be safe to fire, and 2) it is a one-off, and he is usually pretty busy, as are most great craftsmen.
If Frank won't touch it, he might recommend someone who does not mind the liability if it blows up in your face. Joking, but not really. Think about it: they guy doing the reaming assumes a certain amount of product liability if your experiment fails. Tell him John recommended you call. That will at least get him to the phone, but he will make his own assessment.
Also, AR15barrels.com. Randall runs that company. This might be a little too experimental for him. Other names that come to mind are Craddock Precision, BAT and Wilson Manufacturing. The folks at Criterion are good, but they will want a minimum run. Good for production, when you are ready.
Good luck.