Thanks guys! This is the closest so far
http://www.burley.com/home/bur/page_455/tailwagon.html, but a Burley rep told me it's not designed for off road and that "there is no kit for a 29er". Maybe I'll get it, throw some larger wheels on it and do some DIY mods to the hitch... I have to believe that some 29er riders have had a similar towing interest. Holler at me!