Did you see my letter to Cindy Sheehan? Thought I would ping you to it.
First, it wouldn't be a case of dancing on his grave if this really were a "wrong war for bad causes." It's not that, of course, but for those whose war "news" comes from the MSM, her protest might seem to have merit. So you need to focus less on her personal motivations, and more on this particular aspect of her protest.
Second, you might want to mention that members of her own family have asked her to STFU. This would strengthen your case for saying she's acting in opposition to what her son stood for.
And, of course, there's the likelihood that Ms. Sheehan is a willing dupe for the Michael Moore types who are pretty shamelessly exploiting her grief to further their own agenda. Wouldn't hurt to toss that in, too....