PostgreSQL LDAP authentication
I'm not very familiar with LDAP (beware of understatement!), but I'm pretty sure there is no way to do this automatically on the PostgreSQL side.  If you create your LDAP users in a GUI then probably you want to set up a cron script to export the users to the database (I don't know if you can define trigger-like behaviour there, I mean something which would fire a user-creation DB script once a user is created for LDAP).  If you do this from a script then it is quite easy to solve the problem, as you already mentioned.