I have a "stereo" MP3 with audio only in one channel, which is a bit annoying. Is there any way to losslessly turn this into a mono MP3 so the single audio track will play through both sides of my headphones/speakers instead of only one of them? I could always decompress to WAV, save the audio-containing-channel as a mono WAV in any audio editor, then compress back to MP3, but that'd introduce additional quality loss.

A long time ago I looked into this. I came to the conclusion that it's probably possible, but I didn't have time to code something to do it.
...ummm, and I still don't.
But just so you know, I'm pretty sure it can be done.