It seems that the grass type to use for a terrain is attached to the grass output node, and it is a material property, not a material instance property. The limitation there is that if I want to use the same basic material for multiple landscapes (tweaking various instance parameters), they must all have the same grass.
If this could be a material instance property instead, then each instance of my material could have its own kind of grass, which would (no pun intended!) rock for making different terrain types.