Interesting question. AFAIK round rocks would work better than flat ones, but I don't think the hard surface would be a problem.

IIRC there is a relationship between the depth of the diffuser and the lowest frequency it will work on, so that may end up as the limit on what you can do with a rock wall.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8