Hmm. tough call. First, are there any feeder roots coming out of that main root in support of the sucker? If there are, maybe you can get away with digging it up. then you have nice nebari on the sides but it looks as though no (few) roots on the other two sides. If you layer from the root "sides" you still have a lag in nebari development on the other two sides. But if you air layer it right above "the root" then you could have an even start in nebari development. In any case, you'll have to find a way to disguise the layer(s) so passersby don't mess with it for the couple months or so it is cooking.
I think if it were me, I would maybe try a Ground Layer on the exposed roots to see if I could get other roots to shoot in a season, then sever it from the main root after dormancy.
Cool find, though! Glad to see others check out trees along the side of the road and in landscapes...phew its not just me...
-Wm