zen

You might be able to get a rock that "kinda" fits and then rebury the base,(carefully scrape away some of the outer "bark" on the root, right where the base splits, but only down to the green cambium layer and dust with rooting hormone,so that it will send out new roots that will fit nicely over the rock) in something like a plastic nursery can or gallon milk jug with the top and bottom removed, up to above the rock.
Give the roots time to conform to the rock I'd say at least 2 full growing seasons (about 2 yrs.).
When you get ready to expose the stone and roots beneath, do so over the next 2 yrs. about 1-2 inches at a time so that the roots will have a chance to lignify.
You might even want to wrap something like sports wrap (some folks have used pantyhose) to really hold the roots against the rock in the spot where the split is. This will help to get to any new roots to "fuse" with the existing roots on the inside of either side of the split, giving an appearance of greater age and maturity.

Of course waiting 4 years is a while just to see if it even worked,...you probably have about that same amount of time to wait on a decent nebari if you air layer,...so it's really up to you, on whatyouwould rather wait for.
Of course if the "root over rock thing" doesn't work out you might still have another 3-4 yrs. in developing roots from an air layer then.
Patience pays off in this artform/hobby, though and if the root over rock does take and look nice you could have a very nice, and more unique tree.

P.S. Good Luck,....any pics?

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"Although profoundly "inconsequential," the Zen experience has consequences in the sense that it may be applied in any direction, to any conceivable human activity, and that wherever it is so applied it lends an unmistakable quality to the work."

~ Alan Watts (1915-1973)

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