> rather be able to 'unlock' extra tones deeper than the open string

Doesn't down-tuning and then using one of these accomplish that?

I use some chord fingerings that leave the low E and A strings open and form the rest of the chord with the D, G, and B strings. Chords with intervals that start in the second octave sound really dreamy and are easy to play. There is no way to extend that to more than two bass notes without something like this. Four bass notes open up much more interesting chord progressions.

"But it goes up to G" "Can't you just CAPO the third fret?" "...it goes up to G"

Not to discredit the invention, it's very cool indeed.