As a matter of fact... I just sold one of the "surplus" golf cart controllers to a guy in Texas who is building an electric pusher for his PT Cruiser. If he builds it, he'll be the first of many people who've wondered about this idea (that I know of).
More pics of his progress in the builder's Flickr album:
http://www.flickr.com/mbarkley
I'd think the best application for something like this would be for low speed "EV mode" and assist.