My apologies, Jeannette. "Formant filter" was a little misleading on purpose, more like bait. But now that a couple people have bit, i'll be more accurate. Ralf's link to organ stops contains a good demonstration of a vox humana:
It does change formants slightly, but nowhere near as dramatically as the basilica organ. So then, how did the boys from Weingarten accomplish such a drastic change in vowels?
i've found a highly technical article on reed pipes. It may be above my head, but i'll try to read it before forwarding the link.