The BIG and Littles sound like a great idea !
Imex has a couple tires that could easily pass for a ATV tire for the rear ?
http://www.imexrc.com/servlet/the-18/imex-tires/Detail
I have a set of these for my Erevo and they are great tires !
http://www.imexrc.com/servlet/the-16/imex-tires/Detail