I think I'd hire a Formula 1 pit crew. Probably only need three guys (one wheel's worth).
Whenever the Photog yells "Switch", the crew snaps into action.
First guy grabs hold of the lens.
Second guy pushes the lens release button.
Guy #1 twists the lens off the camera.
Guy #3 mounts the TC to the lens
Guy #1 mounts the lens'TC combo onto the camera
With three, you can probably get the switch done on a 400 f/2.8II in well under three seconds...
(You do know the mention of this "sleight of hand expert" in the other thread was meant tongue in cheek, right?)