I do not think HC will grow well on rock alone.
You can try spraying it with a hydroponic solution.
But this must be done frequently.
Or you must use very porous rock.
An idea that might appeal to you: use ADA aqua soil and make a rock ball shape that you want.
ADA aqua soil(ADA AS) is a fairly dense clay, so you can shape and mold it into any shape you wish.
Small emergent layouts are popular here and in Asia. They simply ball up ADA As and add plants, this grows out and the tank and design is done.
This works for terrariums and submersed aquatic plant tanks, as well as this method.
You simply place the "ball" of plants on a white sand base and they grow out.
Looks nice, easy to do.
Solves any "growing on rock" issues as long as you have some decent ADA AS available and can make the shape you want easily.
You can also add balls of planted ADA aqua soil onto rocks independent of the sediment base, eg, up on top of rocks, or driftwood etc.
You can be very creative with ADA AS, more so than Amano has been.
It's clay after all and can be made into many shapes.
This is one reason we can make nice steep slopes in aquariums that stay out, instead of sand which often ends up giving you "flat tank syndrome"
For aquascaping and for folks who have a good idea of their layout before hand, the ADA aqua soil is really good. If you like to top plants vs moving them around or uproot them often, ADA's AS is very useful.
Regards,
Tom Barr