Should be pretty easy to do by sending login/authentication parameters to the server onConnect event method through the client side NetConnection.connect(url, [params]…) method. This post covers how to do authentication against a database:
http://community.wowza.com/t/-/113
I believe Java contains the code to communicate with an LDAP server. If you do a little Googling around you should find some sample code. Here is an interesting looking article:
http://today.java.net/pub/a/today/2006/04/18/ldaptemplate-java-ldap-made-simple.html
Charlie