Root Over Rock. Actually relatively few species are ideal. The reason is that only a few species will fuse their roots solidly to the rock.
For example, Juniper are very slow to fuse their roots to the rock. Most of the time the juniper ROR I've seen the roots are draped over the rock but have not fused to the rock. None the less, juniper get used frequently, even though they are not the best at fusing to the rock.
Maple, especially Trident maple, will fuse beautifully to the rock. Most other maple species will too.
Ficus - of course, famous for fusing to what ever surface the seed lands on.
Pines, - in general most pines will eventually fuse to the rock. It takes much more time than Maples or Ficus, but the effect is good.
I've seen Pyracantha, there is a spectacular ROR pyracantha at the Chicago Botanic Garden. Photos below.
Azalea fuse nicely to rocks. Satsuki and kurume types.
Most members of the apple family, crap apples etc. Fuse nicely to rocks.
I've seen Thuja in the wild fused nicely to rocks. Kimura often does Chamaecyparis obtusa (Hinoki) on the rocks. ROR too.
That is all that comes to mind. I'm certain there are more, but I'm actually drawing a blank.
Key is to start the fusion while the root system is young, usually with a younger tree or with a freshly air layered tree.
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