Now I don't remember exactly - I worked on that a long ago - but there is a policy file written and compiled in Prolog (?) somewhere read by ohmd - and it's responsible for that.
Long story short, what I remember is that I resigned after a lot of searching. Now I am really short on time for next month, but if you are interested in looking at that I can look for my knowledge and send everything to you - some material found on wikis, irc logs and the like