I think that the fire would be way too unstable to control the water temp. The air around the pot wouldn't provide a good constant control to the water. In a crock pot, you got the water in direct contact with the heat source so it can regulate it pretty well and get it up to temp pretty quickly. Seems like the fire would overshoot and have a REALLY hard time maintaining it. I think you'd be better off doing the old cooler method. I've read about it here but never tried it, but seems to work well from what I've read on other forums. Be cool to try that though and see if it might work.
Beer Cooler Sous Vide
Weber Spirit E-310, Weber Performer, Weber Smokey Joe, Mini-WSM, ECB Electric.