Sweet - a kind soul from the mailman-users list provided me with the answer to one of my MAILMAN/FUD questions:
http://www.mail-archive.com/mailman-users%40python.org/msg17520.html
The answer is to do it like this:
PRIVATE_EXTERNAL_ARCHIVER = 'cat > /tmp/mailman_pvt_tmp$$; archiver-1-command
%(listname)s-stuff < /tmp/teststuff$$; archiver-2-command
%(listname)s-stuff < /tmp/teststuff$$; rm /tmp/teststuff$$'
This isn't elegant for the long term, but at least now I can go ahead and continue testing FUD and migrating my lists into it. The only problem that remains is that mail to unknown forums is being treated as an error instead of being imported into a holding forum where I can deal with it manually.
This should be possible via maillist.php - can you possibly give me some guidance?
Cheers,
Tobias