Hi Cordon,
I am almost positive you won't want to hear this. However I'm going to give you my 2 cents worth.
I am of the opinion that to thicken your roots up faster you will have to bury them again as expose roots won't thicken as fast. I would tighten down the roots again (as you indicated) using split shot and cotton kite twine this time to really get the roots in close to the rock. The twine will rot under the wrapping and save you from having to expose the roots to stop the root from girdling. Re wrap it with Keto (muck) and this time bind it with taffeta instead of grafting tape, raffia, or hemp. The Taffeta is soft and will not stretch as much forcing the roots to spread side ways which in turn will do two things. One, give you a more visually substantial looking root system by them having width instead of height. The second reason for the roots growing horizontal is that they will bind into the rock better as this is the only direction you are letting them grow.
As I said my two cents worth. You are on the right track for a nice root system. Good luck.
