I know how to get a zipper for a static graph by Tying the Knot (http://haskell.org/haskellwiki/Tying_the_Knot). But writing one where you can add and remove nodes and edges seems much harder.

You already know how a zipper for a tree looks like. And you have probably seen a functional queue. Generalizing from both of those, you can imagine how a zipper would work.

I can write down the types and all, even if I don't know of any implementation. But that's probably a topic for a blog post, or so.