I'd say I'd have to agree on not airlayering. The air layer would take two or three months, delaying your work on the lower (much more worthwhile) trunk. The section you're talking about has very little interest and little taper to speak of. Additionally, any air layer would take at least three years in the ground to produce any sort of trunk buttressing (which is what makes BC worthwhile as bonsai).
The soil the tree is in looks pretty concrete-ish. I'd be more concerned about getting the tree into appropriate soil at this point.
If this were mine, in the next two weeks, In one afternoon, I'd chop the tree a little more than a third of the way up--allowing room for new buds to develop, remove the tree from the current soil, washing the roots out and then chopping the rootmass to fit a container that's about half as deep as the current one. Fill that container with basic bonsai mix with a little added organic--chopped sphagnum moss, composted mulch, or even a little bagged top soil...
You have substantial work to do on the main tree without worrying about an air layer...