Whelp, I found it, thanks to the info in this thread. The master cylinder O-ring was indeed the culprit. Spraying brake clean around the seam did nothing, but I reached a point that it couldn't be anything else, so I ordered a new one and got the master off on lunch.
It was way worse when I took it off, but the o-ring had gotten caught as the master was slid into place. This most likely happened when the clutch master was replaced by the shop I had to take it to.
The pic shows the two places it was snagged still deformed enough to not lie flat.
Fired it up with the new one in place, and no hissing sound. All I have to do is bleed it with my new Motive Power pump, and done.