Is it an orientation-agnostic evolution from "pre-slash," needing no canonical validation, or does it presume a canonical relationship on the other side of the fanfiction?

My experience is that it requires nothing but authorial intent. But mileage varies, I suppose.

I'm interested in your experience that UST "doesn't have to" "admit emotional entanglement"!

DC Comic fandom was my playground for most of the years I was on Livejournal. There is a high percentage of non-emotional pairings over there (or people pretending not to have them) in the fic and art. So UST was commonly used on pairings based on "pretty/sexy" without necessarily needing to take them into "True Love" territory. (This may also have something to do with antagonistic pairings having a firm foot in the door.)