I guess it's easy to take pics of cosplayers without their consent, but I think it's more fun if you actually get in front of the camera once in a while and interact with them. With that in mind, this will be my personal guidelines for trying to complete the cosplay scavenger hunt:
1. I (or my teammate if I have one) has to be in the picture with the cosplayer. If it interrupts the pose that the cosplayer prefers, maybe have him/her hold a specific prop like a fan in the pic.
2. I will have to come of with some means of remembering what character the cosplayer is dressed up as (write it in a notebook and place a timestamp in the pic since con attendees are not allowed to carry signs).
3. I will give my meishi (name card shown above) that contains contact information to the cosplayer. If you've ever got one from me and wondered where I got them in such an odd size (about the size of an Altoids gum tin), I got them at http://www.moo.com/. They're a really good deal. Better now that Moo sells Mini-card cases. No more putting stickers on mint tins!
4. Full points are given to the first instance of a character. Each additional instance (say in a group photo) is only worth 1 point. Even in a group photo, Rule 1) applies. No points are awarded for any other pics of the same cosplayer in the same costume.
5. While I am tempted to use the scoring system my high school physics teacher gave for tests where you got points based on what elements of the problem solving process appear in your answers, I will not assign points based on what elements of the character appear. For example, full credit is awarded for a pic of Haruhi Suzumiya whether or not she has the Brigade Leader armband, headband, etc...There just has to be enough elements to clearly indicate that an otaku can clearly identify that the cosplayer is indeed Haruhi Suzumiya.
6. If a cosplayer is doing a mash-up of characters (such as Lucky Star's Konata Izumi cosplaying as Miku Hatsune), total score for the pic is half the point value of each character represented.
I'm not sure what other rules should apply that are not common sense (like don't break the rules of the con in order to get the pic, etc...). But I guess this is a start.