No experience, but having taken Statics and Dynamics in college, one point of support is fine for short cantilevers, but for longer ones even a small down force will over come surprisingly large clamping forces. Take as an example the center lug nut on a 1960's VW Beetle. My dad one time had to get one off and found that it was frozen... After breaking his 1/2 Craftsman breaker bar and having it replaced with a 3/4" breaker bar, he slipped a 6 foot iron pipe over the handle of the breaker bar and stood on it - the nut did not break loose, the car tipped up instead!
Based on my experience with my 5" bench vise (you know the kind that sits on top of the bench and swivels on it's base), I can clamp, for example, a piece of copper pipe in the vise to saw it off, but if I try to get the saw more than 3-5 inches from the vise I have problems. If I have more than 3-4 feet of pipe I find I have to support the other end or I have troubles... Not exactly the same thing but my gut feeling is that you will be happier with a dead-man than without.
Chris