I don't see how you could do that without a lot of fiddling, but you can easily blot symbols out by moving them to a separate sheet that has a transparancy effect with a suitable texture overblend. Like so :-
Here's the after shot, with just a simple blur setting (radius 2 on it's own layer and sheet). I also burned the trees by just using a grayed varicolor symbol.
Comments
Well done though, it looks exactly what you wanted.
p