You wouldn't key anthing. You'd simply completely replace it and that can be done most efficiently by applying some rough animated masks around the sand areas.
Grass is not colored uniformly enough to allow good keying in most cases. I see that you're trying to avoid using the green screen, but you're probably going to have to. You could give it a quick test though and see for yourself.
How did you end up solving this issue? Keying? Masks? Please let us know. Sounds like an interesting project.
Wide, stationary shots which involved rotoscoping, and then for closer up shots used real sand...