I think cuttings are a better bet than air layering, but I'm sure some have had success. In any case, I'd save air layering for early spring.

Here is a link that might be useful: Crape Myrtle care

Spring time is your best bet...crape myrtle air layers easily. Use the ring bark/root hormone/sphagnum moss/plastic bag method. If it's a small branch (less than one inch) you might still have time in Austin this season. You don't have much to lose if it fails.
John

I know it's a response years too late, but I thought I would add to this for the sake of anyone researching in the future. ;)

I have a large crape myrtle that I took an air layering from. I used the ring method and wrapped int in a split nursery pot filled with soil. I've read that sphagnum moss will work well too. In about a month or so I had a nice bunch of roots. My new layer has long been removed and the new tree has done very well.

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