OpenLDAP as proxy to Active Directory backend

Goal: LDAP server in Internet facing DMZ to provide authentication for externally hosted applications using internal AD credentials. I've done a LOT of reading and testing, and there is one thing I am still not 100% clear on: Is it possible to do this WITHOUT having a local user database on the OpenLDAP proxy? We will have thousands of users that will need to authenticate, and I can't maintain another user database (adds, removes, etc..). Is there a way to make OpenLDAP just act more like a reverse proxy and forward anything that matches a specific domain on to the internal LDAP/AD server for password verification?