Procedural Foliage Spawner as Landscape Grass Type?

This isn’t exactly the solution you wanted - however I use a mask (eg greyscale png file) as a template for both the landscape grass output node and the procedural foliage spawner; I associate a layer type which I then reference using the inclusion layer in the static mesh foliage type. I had hoped to be able use this as a common mask for both; but it doesn’t seem to work that way. Instead I have to add a layerinfo data type to the layer and then rather than paint it manually, I import the .png file directly into it. It’s clunky, but at least it means that I can have exactly the same template to drive behaviours of trees as well as the ground cover/foliage.