If this were a commercial project, I would definitely explore Mail::Box. However, as this is just a one-off for a friend, I was hoping to avoid having to learn that module or, for that matter, SQLite.

By poking around in ~/.thunderbird, I've learned that Thunderbird stores most of its own data in SQLite3, but the Mork format which Corion mentioned. Again, more than I really want to learn. Fortunately, the contents of any given subfolder are stored in a plain-text file whose path looks something like:

~/.thunderbird/NNNNNsNn.default/ImapMail/mail.pobox.com/INBOX.sbd/subfolder_name

... which is what I'm now playing around with.