Update: so I did this in a fairly horrible manner. I used the mbox importer to import them into a new staff-only category, and then went through them and cleaned them up by hand. Painful, but at 142 mails, not too big a job.
Biggest pain point is that using the mailing list mode archive means every email is <noreply@your.discourse.site> so all the posts get assigned to the same user. It’s also a pain because it (obviously) can’t handle posts that should be part of an existing thread.
Overall, it’s been a nasty job, but the excellent moderation tools (Change Ownership and Merge to Topic especially) make it fairly straightforward.