My workaround has been: if I want to complain by email about X to politician A, I add a politician B who agrees with me on X in the CC, who can potentially act as a witness.
But there's no guarantee that B's staffer will not redirect my email to /dev/null, or that B is not a buddy of A.
Maybe a better solution is something like http://alaveteli.org/, the FOI server software that allows people to send public FOI request emails to government departments and receive public replies from them. So one could put a CC to a designated email address which will publish the email online, and as added benefits others can adapt from these emails. I wonder whether such server software already exists.