I have this problem resolved. It isn't related to the OpenLDAP code at all, but has to do with the password formatting.

What I found was the passwords in OpenLDAP were in this format:

{MD5}<base 64 encoded md5 digest><newline character>

The base64 encoder on the Linux server always adds a a newline character (\n) to the end of the encoding. Multiple platforms have always ignored that character until OS X 10.10.5. Simply removing the newline before inserting the encoded password into the OpenLDAP database allows 10.10.5 and later to authenticate against that password.

-- Jon