The thing is, you need enough bits for that kind of treble extension. With the CD brickwall of 22kHz there's no point in going past, well, 22kHz, and in fact may be a bad thing if you do if the DAC sucks. But I would be very curious trying supertweeters out on hi-rez (outside of actually bringing out the top audible octave).
A supertweeter used... outside of a closed headphone? That's pretty out there, I must say.
If I understand correctly, the smaller the driver (horizontally and/or vertically), the higher the frequencies the driver will start to beam. The ribbons I see are typically long but narrow which means they have good horizontal and bad vertical dispersion. The fact that the full-range panels are so big on my Acoustats mean they beam starting from a very low frequency.
Here's an example of the opposite, a short ribbon with good vertical dispersion: