I think I saw AWUs with Trail Loops when I first checked the Apple website right after they went up for sale, but they disappeared really quickly. Odd fact (maybe): delivery dates for at least some of the AWU/Trail Loop combinations are running well ahead of the delivery dates for the same size/color Trail Loop. So if you haven't already purchased the AWU and you want the Trail Loop band, check out the delivery dates for what you're looking for.
In the meantime, the 44mm/45mm Sport Loops (regular and Nike versions) fit, work and (IMHO) look great on the AWU, though I don't think they'd provide as much security, if you're doing things that might put a lot of inertial energy into your AWU (someone reported boxing while wearing an AWU). The Trail Loop has the same titanium strap ends that the other AWU bands have, compared to the molded-on plastic ends of the regular Sport/Nike loops, and it has a secondary "locking" arrangement.