Out of curiosity, did you have much luck using Eudora's IMAP to do the export?

Yes, actually. I could (in Eudora) add an IMAP account and drag individual messages into it, and then later launch Thunderbird to pull from the same account. I am less-certain about issues around nested folders, which seem to be a magnet for oddities when it comes to e-mail clients and servers.

One of the reasons I kept looking for other approaches is due to how Eudora handles attachments. In this case, many of them had been "detached" from the message they came from. In other words, the message bodies no longer contained the data, but instead a placeholder pointing to a real file in a massive Attachments folder. (Not sure exactly how it handled name collisions.) Uploading such a message to IMAP would leave the attachment(s) behind.

The linked version of Eudora2Unix (unlike the Sourceforge version) tries to reverse this process and reconstruct the original messages, encoding and inserting attachments back into the body. It doesn't always work, but I suspect some of the failures I see are because the file content was deleted over the years by the user to free up disk space, without removing the e-mail trace.

Another plus is that I could just grab all the files for the user and then promise to come back with something better, as opposed to babysitting the upload/download process.