Leonard, I feel you are overthinking this. I have the whole tree and rock buried under 20 litres of my standard bonsai soil mix. The tree is only secured to the rock as it would be to a pot. I have used masking tape to hold the root ends close to the rock and to help the roots go around the sharp corner.
I will let the roots find there own path around and down the rock. I will do this by watering like I normally would this year. next year I will water the tree 20-30% less then I normally would and probably even a little less the following years after that. Using so much soil will stop it from getting to dry and I will soak it occasionally. I will make the roots chase the water down to the bottom. I might go so far as to put a ring of some sort around the trunk and only water and fertilize into the ring to keep the water and Fert as close to the rock as possible. The roots will follow the water.
After 3-5 years of letting the roots grow as they choose (with me nudging them in the direction I want) there should be plenty of roots near where I want to choose from. If not, I'll leave it another couple years (I will dig the dirt off the top regardless to lighten the weight, pruning sideways growing roots as I go until I am near the bottom or the roots get too small). If I do have the roots I need I will pull the tree and rock out, root prune any unwanted roots, tie down the remaining roots with cotton string and wrap with moss and plastic. I'll put the tree back into its container (maybe the ground if I can) sitting so the top of the rock is just above soil level.
If the roots don't adhere in another 3-5 years I will break out my selection of diamond bits and blades and make grooves/indents along where the roots are growing. When I put it back in soil I'll wrap as last time and I will put a very large board underneath with a slot cut cut into the center that will run parallel to the rock making the roots grow under the rock and through the slot. I might even put a thin metal plate flush against the underside of the board that I can close off the slot with after a couple years to sever the roots with and get them to continue growing to the other side. But we might have nanobots I can get to do it by then.
Everyone thinks I'm ridiculous for trying this, and nothing wrong with that. I consider myself ridiculous at times too! I am well overdrive years, a hundred hours and a $1000 in on trying to make a perfectly clear sphere of ice. It's less about the end result then learning the techniques.
Like the one Master Kimura built.
