I have found that several of my fish have chosen certain areas containing plants as their territory. The plants are dense, and form borders much like rockwork does. So long as the arrangement isn't constantly changed, I would think them to be equal to caves.
As far as stocking levels go, I would imagine it would depend on the Africans in question. Mbuna need to be stocked at such high levels I would think a densely planted tank would be comparable to one with a lot of rockwork. Moving the plants around would probably not be a bad thing, preventing the fish from establishing permanent territories. With Haps or Aulonocara, the rockwork is an afterthought to swimming space. In my case, Tanganyikans typically are a bit less dense fish-wise, and seem to prefer a set landscape and small but well defined territories. If planted correctly, I think that it could work with a minimal amount of rockwork, so long as there are breaks in the line of sight between the fish.