It was probably tossed around, but did you guys consider diesel electric as a potential drive train?  Pardon the pun.
I haven't thought about it much other than you need a fairly constant torque output.  Its a fairly typical propulsion mechanism for trains and ships and diesel generators are readily available.
Some reference material for you based off our discussion last night.
Wings and Wheels
www.wingsandwheels.comTerrible website design, but he's got most everything you might need in the way of rope and weak links.
Google Apps:
Document sharing, email, web sites, etc...
http://www.google.com/apps/intl/en/group/index.html
A potential resource is also one of our club members.
Dr. Stephen Schery with New Mexico University.  He was actually doing the towing last weekend for Andrew.
http://infohost.nmt.edu/~schery/