Modify the grass material. Add an RTV to it.
Change the UVs.
This can be either landscape wide (theres a specific formual with bounds to do so, fiddle a bit and youll find it). Or location based if you use an MPC to move the x/y position of the texture around. Either way positioned with WPO so not super cheap.
Pin one of the texture output into the alpha option.
Then at runtime you write to the texture and it will remove (disappear) the grass.
Usually, the RTV is the same used for landscape deformation or grass trails, so its one system levaring the other in hopes of reducing costs.
You can find the “origin” of this in a talk that shows a laser altering the look of the landscape at runtime. Ita old, but its a decent video to start adding runtime effects around…