Air layering would definitely promote aerial roots, on species that produce them, of course. It may produce so many roots, however, that you will have to pick out the strongest ones. Make sure whichever ones you keep stay moist: it would be a good idea to wrap them with moss, or constantly mist them, or else they will dry up and fall off. Good luck!

Would putting my air layered tree in a bag with a humidifier let the roots stay moist enough not to die? Just a thought.

The best way I got mine to throw aerial roots was to repot (cutting roots) in the summer humidity. I guess the aerial roots were a response to the lower water uptake from the then reduced roots, but they sprouted all over the branches.