Brian Van Fleet had a trident maple root over rock project going on a few years ago. I'm not sure what the status is now, but I found the two blog posts he made showing the work. It may help you out.
https://nebaribonsai.wordpress.com/2013/04/13/root-over-rock-trident-project/
https://nebaribonsai.wordpress.com/2013/08/24/root-over-rock-trident-maple-project-part-2/
Right now I don't think your project is going to be very convincing. The bend in the trunk looks very unnatural. A roundabout quote from John Naka:
A cascade should have twists and turns and ups and downs. It should never look like a man’s urine stream; a gradual, even arc falling into the bowl.
(Got that from Adam Lavigne's blog). The initial bend in the trunk of a cascade is usually pretty sharp, not a big arc like yours. I like your idea, but I think that you're trying to force a tree to fit your idea rather than embrace what the tree already is and has the opportunity to be.
Sorry if I sound like a downer. I'm not trying to rain on your parade, just offer some alternative criticism.