Keyser makes a capo just for emulating the Drop-D tuning - I suppose you ought to call it Drop-E when you capo 5 strings up instead of tuning one down. They have an actual U-shaped cutout to miss the low E string. I have a couple, they work pretty well. I know bigchuck has 'em in stock in his store, and would probably mail order you one if you can't find 'em anywhere.
Usual disclaimers - well, I may use my promotional activities for the store as an argument for discounts, but that's as far as it goes.