The best grip for a drop shot is the continental, but you may also be able to get away with it with an eastern grip.
If you're looking for disguise, check out how quickly Nadal goes switches his grip to hit a dropper. It's probably a good example for you since you use a westren grip. (The video is in spanish, but you can see him demonstrate the action in the beginning.)
http://www.veoh.com/browse/videos/category/sports/watch/v20696919y4KPKXd9
IMHO -
disguise is a little less about the starting grip and more about selling it with your body position, then making the quick switch as you hit the dropper. Make the other person think you are going to kill the ball then - DINK.
I saw in instructional vidoe on the tennis channel where the guy say you should even make a face like you are winding up to hit it hard- LOL!