There is something like that in CryEngine2/3, and there was no noticeable rendering cost for coloring grass to match underlying terrain color.
Though, my bet is, to make it really performance friendly, you would need to create custom shading model in HLSL, with access to Landscape textures info, and sample colors from here, to blend them over grass.
A task, for someone, who is good at writing shaders and understands unreal rendering ;).
I’m not sure if material editor have enough information exposed to it, to make it work.