Interesting. I'm trying to figure out the logistics of it. The current usual seatbelt setup is the belt anchored at one hip, a free sliding latchplate, and retractor at the shoulder. In order for the airbag to go from the buckle to the shoulder, I can think of two ways that would work. With the usual sliding latchplate, one end of the airbag is actually attached to the latchplate. The airbag is then on a sleeve that goes over the seatbelt, and slides along with the latchplate. The other configuration that I can come up with is to have separate belts for lap and shoulder portion, either with a sewn-on latchplate or a detachable shoulder belt.
With harnessed seats, the airbag would deploy in the belt path, and that wouldn't be safe. They could take care of that problem by providing a way to turn off the airbag, or by using a detachable shoulder belt. Then the carseat would be installed with the lap belt only and the shoulder belt (which has the airbag) would not be near the carseat.