I like the idea of a clamp that cab be attached to a number of surfaces. Does anyone here use such a device? What are your thoughts? I'd be worried that it couldn't take the weight of a 5D and would loosen and fall.
That wold be exactly my worry too. I have seen pro-quality suction cup solutions that I would trust at reasonable prices but those probably don't have the mounting flexibility you are looking for - hmmmm, interesting question you pose.
I would tend more toward the Manfrotto (and other) Superclamp - they come in a couple of sizes, and they lock onto just about anything: doors, poles, small children. They will hold quite a bit of weight securely.
They are made to take standard lighting fixtures in the socket, but there are all kinds of adapters (they often call them "spigots") that make it able to hold 1/4-20 mounts - it actually has a 1/4-20 female thread on the front. Here's a particularly ingenious one I just ordered from Hong Kong (I have yet to see them in the states) that gives you a whole bunch of mounting options: