Inspired by the proprietary server at ldap.dnssek.info, I'd like to make a slapd plugin that, when queried for a particular email address, finds the OpenPGP keys and S/MIME certificates by doing DNS lookups (possibly aided by DANE), and then serves them back to the requestor.
Interesting. Although I am not entirely sure what you are trying to solve and why this needs to be solved in ldap and not in eg. MTA