Look down at the bottom of the page for turlough's silk-icon adaptation. It's a lock+a tag, sort of like LJ's double-locks but (I think) even more intuitive than the double lock, because it's saying it's a tagged-locked post, which if you think about it, very much fits what filters are: they do get their own tags in the post info, after all.

So if we were voting for icons, that one would get my vote, plus it's using the same icon set as its basis, so visually it's also seamless.