[Resending because I forgot the ITS number in the subject]
I have a simple prototype that on add/modrdn populates/maintains an attribute (rdnValue) with the distinguished values of the naming attributes of the RDN. While doing this, it also checks for uniqueness of the rdnValue value within siblings.
You can find it here ftp://ftp.openldap.org/incoming/pierangelo-masarati-2009-08-03-rdnval.2.c
Please test and comment. I hope it does what intended.
p.