Over the weekend I gave the smtpinvite thing that I found on TechTV's server a try. The idea is that the primary MX will always defer people - it exist solely for adding sending hosts to a whitelist - when the remote MTA then goes to the secondary it will let the mail through.
This works fine for most MTA's that I looked at during the few hours I had it up. Sendmail however will not contact the secondaries when it gets a defer message it will just keep trying to talk to the primary on each retry! Surely that is wrong, I have not gone to study the RFC's but it seems very stupid to me. So unfortunately I had to abandon the idea.
You'll likely also get false-positives from Lotus Notes, which internally implements a VERP type scheme for retries.