There are three variables here (assuming modern cranks)
I'll start with Direct Drive.
I have no experience of the original 'DD' cranks but I suspect the axle / BB system is a copy of something rather than proprietary.
I currently have two sets of DD2 cranks which are exact copies of Hollowtech II. I've used them with Dura Ace, Rotor and Uberbike bottom brackets.
On to the square Taper SG75.
Like the Dura Ace 7600 cranks, technically these should only be used with an ISO taper. The Sugino 75 Cartridge BB or cup and cone BB are both fine and have the correct spindle and taper.
If you're using 'new' SG75 cranks then I would recommend using one of these or sourcing an ISO BB with the correct spindle length. There are very few around. Some have had success using a Campagnolo centaur (my experience with the Centaur was bad and I returned it)
Using an SG75 with a JIS BB:-
If the cranks are used and have previously been forced onto a JIS taper, the hole will be slightly enlarged and will probably be best suited to a JIS taper from then on.
If the cranks are new, there will be about 3mm less engagement on a JIS taper. As such, you could use a JIS BB with a slightly shorter spindle but be careful not to over tighten them or see above.
Sugino put the compatibility warning on their website basically to stop people forcing new 75's onto commonly available JIS tapers. Although they claim their BB is proprietary, if it is, the difference between it and ISO is small enough to be negligible.
I've just stuck some 7600's on the Sugino Cartridge BB and the union is perfect.