(converting answer to comment since it was a comment not an answer)
Here is a promising new idea that I haven’t had time to try yet but will work in 4.8:
you could try using the new landscape grass system to spawn small meshes that are designed to work with tessellation. You could use world aligned textures for them (including displacement) so the meshes could appear to just add detail to the landscape. but you’d be stick with positive displacement (ie no easy way to lower the terrain) and of course no collision support, but tessellation in general has no collision support. This approach should be much faster and should be able to get similar results.