Creating vegetation polygons on custom 3D models

Hello everyone. I have repeatedly faced the need to add the most ordinary
grass to the scene… There are many surfaces on which this cannot be
implemented. Since grass can only appear on the surface of the earth. It is
often necessary to create a variety of 3D models in which there is a variety
of vegetation. Flower beds, terraces, various horizontal surfaces located much
above ground level, where vegetation is needed… Unfortunately, in the vast
majority of cases, it is impossible to solve this problem using terraforming,
since this method cannot provide the necessary accuracy. And in the case of
creating green surfaces at a considerable height, this method is simply not
suitable for solving this problem. I hope the developers will take into
account my wishes.

Same question here. I have some nice 3D surfaces for airports with drainage
channels and retaining walls here, so very complex 3D layouts that are not
possible using heightmaps nor polygons as their resolution is just not
sufficient and you cannot texture the walls there. Unfortunately is is not
possible to benefit from the default grass that moves with the wind and looks
way better than any custom grass (considering how performance friendly it is).
Unless of course someone has an idea how default grass and vegetation could
still be called on custom locations that are not bound to the terrain?