I have seen the same thing in a french prod's greenhouse: Paphiopedilum lowiis on rocks.
The prod is Nicolas, from "La Cour des Orchidees" and from his own words, result was far from concluant. The plants were growing twice slower than the very same seedlings, growing in bark. He told me he did that as an experiment, but won't lose his time again on it, useless he said.
I will go back there before the end of this month, and see if I can take some pictures of these lowiis if he still got them.
I have to say that it looked beautifull, even if not concluant from a prod's point of view (fast growing, fast flowering).