This is a link to a sample project:
https://www.dropbox.com/s/pp5pb1ovu81oay5/ProcFoliageISM.?dl=0
There is place in constructor where ISM can be replace with HISM.
With uniform scale of hexes, Foliage does spawn on Instanced Static Meshes. For HISM no difference they don’t spawn.
Unnatural pattern in which foliage is placed (grid like) is probably a result of my settings for foliage.