Searching the web for solutions on how to export messages and folders from outlook on the mac, I found a ton of useless articles about olm files and windows articles about psts.
The easy answer was the most macintosh of answers: Drag the item to the desktop.
This creates a very familiar file, but one that is not so easy to deal with in most gui applications, an old style mbox.
This is a single massive file rather than a folder structure, but it's cross platform and usable with unix mail utilities.
https://en.wikipedia.org/wiki/Mbox
It's easy to SCP to a fileserver and simple to then also search with grep. You get all the headers on the email, though most of the contents are HTML these days.