I tried using hypermail to convert some of the mbox files into HTML so I could post them online, but for some reason hypermail doesn't seem to work on any of them. Thunderbird reads the files fine, though. If anyone has any ideas about why, let me know. I could even provide a sample mbox file if you email me (address at website linked in my HN profile).
The worst part is that there's one private Yahoo Group that I never was able to get access to that apparently was important in a hobby I've participated in. I guess that one's going to the bit bucket...
Edit: Archiveteam has information about other archiving programs: https://www.archiveteam.org/index.php?title=Yahoo!_Groups