Dave, this is a tall order. You're looking at taking a 0.0025" DOC over 12" and require a precision fit all the way down the bar. If you have even a tiny bit of deflection of the bar, and you will at that length, then you are unlikely to hit the required ID with the precision you require. Line boring "might" do it if the set up is perfect.
Thinking about it, I see three possible options. One, you could start with a solid bar and bore it with a solid carbide boring bar so that you can control depths of cut. A solid carbide 3/4" bar will get you half way and you can flip it and bore from the other side. If the set up is precisely controlled, you might meet in the middle. Two, you can try using your current bar and run a reamer through it from both ends. Not sure this would work but working with a reamer manufacturer might get you there. Third, what about lapping it to size? I'm not sure how you would do it but I imagine a line boring set up but use abrasive instead of a boring bit. This would allow you to cut and check, cut and check. If the lapping bar is solid enough and you run slow enough, it might work ... maybe.
I've done #1 and #2 but not over a 12" length so I cannot say for sure that it will work. Good luck with this one - as I said, tall order.