Etiquette of publishing folklore results

You ask for the "etiquette", which may differ from field to field. For the research community in computer science, the fate of one particular folk theorem has been documented in loving detail by David Harel in On Folk Theorems (1980).

One thing to note from this exposition, is that a characteristic of many folk theorems is that they are vague ("While-programs compute everything" is the particular example discussed by Harel), so they give rise to more than one version, and each version requires a seemingly different proof. Harel finds over 50 references to proofs of various incarnations of this folk theorem:

These proofs, which span fourty-four years, also span the complete spectrum of recognized scientific literature: textbooks, monographs, survey articles, journal papers, conference proceedings, newsletters, theses, technical reports, lecture notes, letters to editors, and self-referential folk tales.

So I would conclude that for this field at least, publishing a proof of a folk theorem in whatever medium you find most convenient is "proper etiquette".

Include it in your paper, and write clearly that on your opinion this is a known result but you could not find a reference. You can never be sure that this result has not been published by someone, and you just don't know.

Or, you can do as they (Phelps et al) did at U Washington, and publish numerous results under a pseudonym (Rainwater in this case) ....