Graham...There is such a device, though it's not a capo. It's very expensive, and so far I've only seen it rigged to a Les Paul (Jimmy Page appears to endorse it) but there may be applications I'm not aware of, of course. Probably weighs a ton.

As for the virtual tuning capo idea, yes you're essentially correct, once you're beyond open position, standard (or whatever tuning one starts with) is what you're playing. New timbres, textures, voicings are still available based on the different open notes. I like them alot for the drones you mention, modal effects, and "drop" type tunings without the hassle of extreme refingering. I don't think anyone seriously uses modified capos to convince themselves they're using open tunings, nor would I pretend that they're a universal solution to everything, but there are advantages. I often tune low to start with, but even in standard I can get a nice drop D, double drop D, Em, A, dadgad (up a step), and other effects. I think of modified capos as a cool addition to my "trick bag", like slide, E-Bow, digital effects, and various techniques...usually all used sparingly.

I'm a genuine tinkerer, so such things appeal to me. There's no cure.

Peace, Mooh.