An ambitious project for me. It was a lot of fun to figure out. I'm hopeful that it'll grow in and cling to the rock in the future. Getting rid of that wire would be nice.
Once it puts out some growth and shows that it's established, I'll do some semi hard pruning to refine the shape a bit.
The pot is one I formed by hand with a mix of portland cement and perlite dust.
I mostly got lucky with the root placement in relation to it's position on the rock, for what that's worth. There are two pieces of wire that I used to strap the roots to the rock prior to it being planted. Those seemed to secure 3 (of the maybe 5) roots relatively solidly around the rock. After trimming off a few long roots, I tucked the rest down around and underneath the rock. Filling in the rest of the soil was straightforward.
Prior to this planting I had it growing out in a kind of semi-cascade style. It was initially a small cutting propagated in water, transferred to a 4" nursery pot for about 8 months to establish initial structure, then transferred into a 2x deep 4" pot for the semi-cascade, and now the root over rock. Not a ton of root training on it besides making an initial root flare with the early repots. I definitely lucked out with how it joined up with that rock (that I found in the yard).
Ask away if you have any questions! I could probably dig some pictures up, too if you really care.