Hi Sebastian,
This is not a FAQ. Postmodern currently only implements MD5-password authentication. I never really looked into the other methods that Postgres supports. If you want to implement other methods, see cl-postgres/messages.lisp and http://www.postgresql.org/docs/current/interactive/protocol-flow.html#AEN797... . It doesn't say anything about 'sameuser' there though, so I'm not sure what the mechanism for that is.
Cheers, Marijn