Hello, interesting update on this.
I’m a bit OCD wanting to install a NOS Campagnolo MTB crankset on a couple of bikes.
I recently acquired a Zinn bike with an Icarus crankset.
It appears that the bike has a fairly new Campagnolo bottom bracket??
On removing the cranks I noticed that the spindle extends very close to the end of the square taper.
Almost, “ bottoming out”??
Assuming that the axle is modern and ISO; this would suggest that Campagnolo MTB cranks are not ISO.
After reading very limited, unconfirmed, information on the Internet; I think that the tapers are different, but not JIS either.
Below are photos, including comparisons to an MTB centaur spindle (black) and, I think, a ‘road bike’ Record spindle (silver).