It is already possible to generate Octahedral Imposters using Ryan Bruck’s upgraded plugin by Luciano. He made a course about it if you are interested: https://www.udemy.com/course/unreal-engine-octahedral-imposters-create-lod/learn/lecture/22554426#announcements
So far as I understand the limitation with Nanite comes from it unable to handle animation/WorldPositionOffset. While I am hoping there will be improved foliage feautures in the future I am not personally not counting on it. It seems we are still bound to oldskool methods of foliage creation. Keep in mind though that Nanite might open up more rendering budget for foliage so you mind, possibly, not even need to use Octahedronal Imposters.