Thanks for your response, Wolf...
For those not familiar with spider capos, and to illustrate that what I'm talking about is really very simple and not beyond the physics of normal guitars, here is my modest setup, capo'd for F-chord:
As the second photo shows, the 3rd fret can still be fingered if needed. Simply set F, C, G, D, Dm, A, Am, B, E -- whatever you like, then play additional chords and notes in standard positions from the 3rd fret on up.
Unlike many alternate tunings (see 'special case' exception below), chord shapes and scale patterns remain unchanged.
Which is not intended as a knock on the utility or appeal of alternate tunings, just simply to illustrate my hypothesis that in the general case, physics of alternate tunings vs capos is distinctly different.
By inspection I think, these special cases are obviously equivalent:
An alternate tuning where the tension in all strings is changed by an equal amount (e.g. Eb 'blues tuning') is equivalent to
Applying a capo across all strings at the same fret. ...regardless of whether the fret is physically above, or virtually (as with Eb tuning) below the nut.
In the general case however, tuning controls the tension of a string and therefore the tonal distance between strings, while capos control where a string is fretted regardless of tension. As the spider capos demonstrate, strings can be physically capo'd independently while leaving the tuning unchanged.
At this point perhaps a YouTube video is in order to demonstrate the usefulness and fun of this idea.
But first I'll need to get my basement studio-pit tidied-up a bit... give me a week or two on that tall order, the maid's been on permanent vacation!