^ Spot on, Arthur.

Last night as I dropped off to sleep it occurred to me that there may be an even cheaper, possibly lighter, and more convenient way to do this. This morning .... I can't quite remember what my brilliant idea was. Maybe I can duplicate the mental steps and wind up in the same place.

Various answers in this thread nail the major issue. Yes this is possible and practical. Now we get to the important little details.

Mounting to the camera is the first one. The ubiquitous Arca-Swiss plate is actually not ideal. You need a plate for every camera you might use with the rig because the plates mount semi-permanently with an Allen key. This means some extra cost (I'd mostly use the 5D II for this but might want to use any of three others) and (the deal-breaker for me) extra bulk - my camera bag is not really big enough to comfortably accept cameras with plates and I can't buy a bigger bag. (Yes, honestly, I already have the biggest instant access waist/strap bag you can buy.) And then we get to another difficulty: incompatibility with my landscape head mounting, which is the hexagonal Manfrotto type. I could buy a new landscape head (it's just a cheap little ball head after all) but the Manfrotto hex plates are really good because they are one of the few decent plates you can attach to and detach from the camera very quickly and easily without tools. You can just keep one in your pocket and mount any camera, any time.

So, working from that ^ - could we find a monopod head with the same mount? (Answer: probably not. Manfrotto don't seem to have one, anyway.) Failing that, what about fixing a hex-plate socket to any desired head? Manfrotto have a couple of very cheap, light-looking one-way monopod heads - http://www.digitalcamerawarehouse.com.au/prod2784.htm and http://www.digitalcamerawarehouse.com.au/prod1061.htm - all we need is to mount one of these http://www.manfrotto.com/hexagonal-plate-adapter on top of it. Solved!

But wait! Why bother with a plate at all? Why not simply screw the camera directly onto the head each time? It wouldn't take more than 10 seconds or so (which is perfectly OK - it takes just as long to mount a hex plate and pop it into the quick release) and you wouldn't have to do it up tight as this is not the sort of rig where you'd walk around with it over your shoulder and need absolute faith in the security of the connection. Am I thinking straight? This looks like the best answer - especially at a princely $37!

Well, we still have to find a way to mount the head to the rails on the Wimberley. Something like a sort of reverse Arca-Swiss plate would be needed. You can probably buy them, and it's the sort of thing that you could make yourself if you had to.

I have to go to work. Great answers lads. Thanks again!