Persaonlly I would suggest NOT using 389DS. It's bit and old and clunky. I would also think that you possibly don't need to replicate data to LDAP, but could look at a translucent overlay instead. This allows you to effectively proxy AD via OpenLDAP and make a few compatability tweaks to the data as it passes through, including adding groups to accounts and such.
your test solution sounds like you are authenticating against AD but not obtaining user information from it. This could be possible on an ongoing basis by having OpenLDAP hold the posix user account info, and then you can either do authentication totally separately to OpenLDAP or proxy it through for a nicer diagram. http://linux.die.net/man/5/slapo-translucent
Good example here - http://www.openldap.org/doc/admin24/overlays.html
From what you do describe, I'm not seeing a RADIUS perspective, but ultimately you need a legitimate LDAP base in place before that's of any concern any way.