What form would the configuration of <the database with non-UNIX accounts>
take?
# in /etc/postfix/main.cf
local_recipient_maps = proxy:unix:passwd.byname, $alias_maps,
<the database with non-UNIX accounts>
The documentation page http://www.postfix.org/LOCAL_RECIPIENT_README.html seems incomplete in that sense.
An example would be awesome.
It is not incomplete. You just miss this important line
that point to this section.
So the example would be