John - If I understand correctly you looking to use the advanced tab of branch rule to determine how to route a user. This is an example of a check for the existence of a value from a LDAP query. You would need to use a AD session variable instead of the ldap session variable but the basic structure of the check to see if a value exists is like this:
expr { [mcget {session.ldap.last.attr.unixHomeDirectory}] != "" }
I've never tried or thought of using the check on a branch rule of the query item itself, but I would think it would work (and I might start using it if it does). I tend to do the branch selection with an item following the query itself mostly to keep operations broken out so I remember what everything does.