The OP appears to be correct in saying that some current Sugino cranks use a proprietary taper.
Some current Sugino cranksets use what Sugino calls a "SG Mighty taper". In particular, their Mighty Tour and Mighty Mignon cranks use this taper. From Sugino's website page
for their current "Mighty Tour 901D" and "Mighty Mignon 901D" cranksets:
SG Mighty taper is a proprietary standard by Sugino, not compatible with other standards.
Photos at that page appear to show it similar to a square taper, but with concave-dished sides.
This taper may or may not be used on other current Sugino cranks. I didn't look in detail.
Edited to add: OP, I'd be very hesitant to purchase one of those cranks without first confirming that a new BB
for your specific frame was available with that proprietary taper - because I'd guess the spindle you have now isn't compatible. And I'd also guess you'll need a spindle length different from what you have now.