Nice, this is going to be cool. As far as the dress, I heard people dig holes and bury zombie attire in the ground for a month or so, throwing random stuff on it before it goes in there like coffee grounds or something like that. Just make sure it's not a food that can rot cause it will be nasty when it comes out. Water it like a plant too, water will help the decomposition. And don't worry about it decomposing too far because materials in those dresses are usually synthetic and it won't do hardly any damage in that short of time.
To help distress the clothes last year I use my electric sander and with a block of wood, I push the clothes on the sander. I put in holes and tore places all over it and then I got a 99 cent can of black spray paint (brown will work also and may look better with both) and lightly hit it with paint from a couple of feet. I think it came out great.
They look great by the way