The RHQ ldap integration assumes that customers will use the concept of LDAP groups as the standard way of organizing and subdividing the set of all LDAP users in the system. If I understand your post correctly, your company has a use case where there is no need for grouping of ldap users and you have a 'flat' ldap schema. Two questions:
i)Can you elaborate a little further on your specific use case? We may need to create an enhancement request for this new functionality if the existing standard approaches are indeed insufficient.
ii)I'm not extremely familiar with the Open LDAP server, but it should not be an ldap schema change to create ldap groups which bundle some or all of the existing ldap users. Have you attempted this with the Open LDAP server? This is the simplest solution to try if you have not already.