Work around it.
Act on the grass instead of the landscape.
Basically. Discard RTV.
Make your own system that affects grass near the camera/player. Using a screencapture2d and a render target.
Like snow trails, or grass deformation.
Then you add the item/rock whatever to the list of things that are rendered in the render target that controls the grass.
Then in the grass material you account for the render target using world position.
And the grass just doesn’t show up depending on what you render/rtv settings.
Costs a bit, but it will always work and also allow for a more dynamic gameplay environment.
RTV can do this too, but it would then affect the landscape paint layers or grass nodes. The resolution on those is horroble for large landscapes if you want to keep rendering costs sane.
I know of no way to take the existing rtv render and use it esewhere.
But its an image and it has to be in memory even during gameplay. If you want to hack a way out of the engine you probably can…