One way to maybe display multiple coatings would be to make custom
tiles for each coating
with holes in them...for instance, for grass make a cloating that is
"patchy", so you
can see the underlying tile terrain underneath. For rain, clouds, and
snow, it could be patchy
in alternating areas, so that you could put a grass coating and a rain
coating together
and have the holes in one merge with the patches in the other.
For thick forest, you could fill the center totally with tree texture,
and leave a couple of
corners open to show other coatings/terrain...