I've added this to the wiki, it's actually the first time I've made a change to one, the interface is somewhat clumsy so it took me a while and if I've not done something in the 'correct' way then please feel free to change it tell me.
Glad you like this method, I dont know postfix at all really, so it's all guesswork to me, but if it works it works
This solution means that mail sent internally does get stamped with the disclaimer, now I currently dont know if that's a good or bad thing, but if it turns out to be a bad thing, does anyone have any idea on how to stop it happening?