On some reflection, the easiest way to get a tree of non-Exchange folders imported into a Zimbra account seems to be by pointing something at them that can also do client-side IMAP.
I expected that search to take forever, but I got lucky on the
first hit.
I have not yet tested that script, and I'm going to look for a few more, but if someone who knows what they're talking about could scope it out for sanity, that'd be real cool. :-)