Thanks for the response, @stephomi. Your explanation makes sense, but I'm still left wondering - is there any way to temporarily provide (perhaps on a probationary per-request basis) a bypass for that particular lock?
As you probably know, one of the current community contests - a hugely popular one, I think - is all about creating immersive movie-inspired scenes with VR particularly in mind. As of the time of writing there are ~50 entries and there's almost a month to go - a lot of people are very interested in making things for it. The end results would surely bolster Sketchfab's VR catalog which - last time I checked - could really use it (~24 or so entries in the VR portal?).
Why not take this opportunity, when so many people are passionately dedicated to generating new VR content on Sketchfab, and provide them with the tools they need to make it great? HDRi environments are one of those things that can make or break immersion. I, for one, would hate to put disclaimers on my scene along the lines of "due to sketchfab's current limitations...", especially if the limitations are the result of a decision rather than a tech gap.
Anyway, just thinking out loud. I'm sure a lot of us would appreciate the ability to do so and make an even stronger case to paying $120 to continue the Pro membership past the contest.
(CC @bartv who seems to be particularly involved with that contest.)