Allright, check this out. I got this book called Winners Circle, it's winning homebrew recipes from the AHA's national competition, and in the mead section there's a lot of recipes from the "early days" of organized American homebrewing using hops. Most of the recipes seem to use about an ounce of hops for five gallons, Fuggles or Cascades. One recipe used Hallertaus. I'd imagine it might work similar for cider, maybe use a little less... Cascade would be the first hops I'd think of using, with that citrusy flavor it might complement the cider flavor well.