Hello Jens,
Postmodern does not really do anything like join-slots. I never found them very workable in CLSQL (possibly because I didn't understand them well enough), so in Postmodern DAO objects are just simple wrappers for database records, and don't do anything fancy.
(Though of course, with some creative method wrapping you could make them do all kinds of bizarre things.)
Cheers, Marijn