Junipers are often used for root over rock, yet they seldom get the fused, adhere to the rock look. I would say junipers are a poor choice.
Hornbeam and Otsrya seem to fuse well. Amir maple will fuse well.
Hinoki I believe would work well, but their a intermediate to advanced skill level tree, not an easy species long term.
Satsuki azalea have a reasonable long history of being used for root over rock.
Key is, to get the fully fused to the rock look, you start with young material and grow it on or with the rock for many years. When matching the tree with the rock, be sure the rock is large enough to look in proportion with the future mature & larger tree. Difficult to get right.