su-like behavior

Hi,

I want to run some sql scripts through psql from a process that runs as
root. Since I have no reason to trust the sql scripts, I want to run
them in psql as a specific database user that doesn't have a
corresponding login account, i.e. I want to do something like

[start as root]
su - postgres
psql DBNAME DBUSER < script.sql

or
[start as root]
su - postgres
psql
\connect DBNAME DBUSER
\i script.sql

The problem is that I haven't been able to do this without being
prompted for a password. Is this possible, i.e. is there a way to go
from a superuser (postgres) to a less privileged user without having to
supply the password ?

thanks,
David