Okay, I attempted to convert the Jay's Journey tracks "Starboard Authority" and "Starboard Authority (Alt)" into OPUS files. They play just fine so long as I don't mess with the loopstart and looplength attributes (which, true to what you said in that topic, do not work properly). If I do alter those attributes, then upon playback I'm greeted with:
Failed to set the 'loopstart' property on 'AudioBufferSourceNode': the provided double value is non-finite.
So either there's a previously-undetected bug in my plugin, or there's an issue with OPUS files.
(Actually, I did discover an undetected bug in my plugin. I'll, uh, just go and release a new version for that. But it's not one related to looping or OPUS files.)