Things I know:
Prog-ish. Not particularly helpful, I know, considering the venue. But I don't think it's down the middle jazz. Possibly fusion.
Single word name. Perhaps starting with an "E".
I read about it in a blog post a few years ago. The collector mentioned it as a "holy grail" because it was actually never released. The blog post mentioned that it was not very good, but I'm still curious to know what it is. I believe they were allowed to purchase a copy with the caveat that they would not distribute / upload it. Possibly from a vinyl shop with whom they had a relationship in Paris.
The Montreux festival database only goes back to 2012 when searching by year, and I don't know the name of the band.
It doesn't appear on any of the pages cataloging official releases, because it was either a test pressing or a bootleg.
Any records of participants in the late 70s / 80s list only the featured artists, not every performance.
It's possibly Etna, but I can find no mention of a lost recording or album of theirs. It's also possible that it's not an E name either.
Any help appreciated!