I wanted to do this technique one of these days. Roots have cambiums, so you can graft them anywhere there can be a cambium contact as long as they’re compatible.
One uses of grafting roots higher up in the tree is to shorten the tree, or to prepare it for moving. Rather than digging it up and balling the tree, simply ball it with the lightweight planting material above ground and not do any digging at all.
Also when one is propagating tree via marcotting, it may be advantageous to graft some roots the spot where you want it, and then later on cutting that branch that you want to propagate.