I have just had another go with the alembic grass feature, and am still struggling to control the result.
It seems like the y size parameter gets ignored when the grass is being generated (reducing it does not seem result in shorter grass being generated), and while rescaling it for already generated grass seems to do something, it also lifts the grass off the ground - so it seems to rescale scale from an origin somewhere above the top of grass rather than at the base.
(note for what it is worth, the proxy object used for the grass is sitting with its base level with the mesh, though I understood the proxy location should not matter either way, and that the origin of each grass object in the alembic file is whatever the surface of the mesh is at that location?)
Also struggling to change the material - whenever I select a new material and click 'ok', what looks like a small dialog flashes for a microsecond before disappearing - and nothing otherwise seems to change.
I am probably missing obvious things, but would be interested to know what they are.