Now we have a new contender:
While poking about in various manuscripts of Petrus Thomae's Quaestiones de modis distinctionum, I came across the following comment in the margin of question 7.
Hoc argumentum solvit egidius in de esse et essentia q. octava qui fuit inventor formalitatum (Munich, Bsb, Clm 26838, f. 34r, al. man.).
[For the Latin impaired: "Giles, who was the discoverer of the formalities, refutes this argument in his work on being and essence, question 8,"]
This is an annotation of the following argument:
Confirmatur, ista enim attributa sive formalitates ut distinctae, vel sunt aliquid et res vel nihil. Si sunt aliquid et res, propositum. Si nihil, ergo formalitates sunt nihileitates.
[It is confirmed, for those attributes or formalities as they are distinct are either something and a thing or nothing. If they are something and a thing, we have what we are trying to prove. If nothing, therefore the formalities are nothingnesses].
Egidius of course is Giles of Rome, who, depending on the decade, is either beloved or despised by Thomists. Thus we have a (quasi?) Thomist to add to the origin story of the formal distinction, which becomes less of a characteristically Scotist position but a tool made use of by a variety of scholastic thinkers.