Will the Koolstop inserts for Mafac holders also work in the Universal Mod 77 holders and similar holders?
The Koolstop Continental pad+holder work but do change the "look" of the brakeset.
The MAFAC pads have exactly the same bevel angle and width as Campagnolo NR/SR, but they're longer. There is no reason to believe that they will fit Universal holders.
You could try to find some Campagnolo holders. Kool Stop pads are available for them and easy to change since one end of the ends of the holders is open. The holders will probably be very expensive. The Kool Stop Continentals will work for you, but I think they're hideous.