This idea was brought up on the Yahoo Vandwellers Group (a great group for the mobile community). Some travel with cats and dogs... now you can have a chicken in your vanstead without fear of a mess:
http://www.chickendiapers.com/Catalog.html. The chicken on the front page of this link is one of the cutest chickens ever!
I also found this site for all your pet chicken needs:
I know a van dweller that traveled with a pet chicken once (in a bus). Charlene reported that her chicken would like to ride on her shoulder while she was driving. She would stop to let it out and she didn't have a mess.
If you're saying to yourself "I *have* to get a chicken in my vanstead", I've found just the group for you: http://pets.groups.yahoo.com/group/housechickens/. I've only been a member for a very short time, but it isn't hard to see that these folks are some of the nicest people you could ask for in a group.