Just to keep anyone looking at this up-to-date:
I've settled for option 2 (installing from source) and fixed the separate-host problem by implementing an ersatz postmap script which calls postmap and then uses rsync to copy the files to the mail host:
#!/bin/bash # in addition to running postmap, copy the files to our mail server /usr/sbin/postmap $1 rsync -a $1 $1.db mail.heeg.de::postfix-mailman3/
Of course this requires setting up an entry in /etc/rsyncd.conf but then it works reasonably well.
The socketmap service is still something I'd like to tackle when I have time.