I think a workaround for this would be to paint a map in a separate unreal map or a third party application. not ideal but actually might give you some finer control over the grass output.
If painting in a third party app like photoshop or even maya/zbrush/mari, you could render a heightmap and then use that as a guide to paint a greyscale texture and block out your grass in the material before it goes into the grass output node.
Or to do it in unreal would be to create a duplicate map (which I assumes saves a different copy of the landscape? i havnt tested) and paint a layer info so you dont destroy your weighting in the main map. then export to file and read it in the material.
I’ll have to try when I get home from work.