Years ago, I had a Vivitar extender that was coupled with a variable extension tube - it was quite useful for macro. I suppose you could say it allowed independent control of magnification.
That's different than what OP was referring to. An extension tube is, as you say, is good for macro photography. Not so good for increasing the focal length which is what OP was talking about. A variable extension tube is pretty simple, since extension tubes are basically just empty space (such as bellows), whereas tele-converters usually contain 1 or more lens elements to re-focus the image to get it to hit the focal plane correctly.
Yea, a zoom Extender, TC - not an extension tube. If not continuous, how about a stacked option - similar to the pending 200-400. Move a different power extender into position. Perhaps 1.0, 1.2 and 1.4 options
From the lack of anyone have seen such a device I suspect that it optically very difficult at best
Well, sure, you can stack TC's now, they just are different physical things. I imagine it'd be quite a bit more complicated to have one device that you introduce the different optics into position. More complicated mechanically, which introduces many more points of failure, and probably much more complicated optically unless it simply lifts one of the optics out of the way and puts another in it's place. Theoretically possible...but why? Just pull off the lens, and stack your TCs.