CCK + Taxonomy + Permissions, oh my...

Hi all,

I have a content type ("Publication") with a CCK file field that allows the attachment of a PDF. For most instances of the content type, this file should be inaccessible to anonymous users; okay, that's easy enough. However, some publications carry a particular taxonomic tag ("Free"), which means the PDF should be visible to everyone. Is there a simple way to make the accessibility of a field dependent on the presence of a particular tag?

(I realise I could just create two content types, one for free publications and one for the rest - but that seems rather clunky, especially if the Free status needs to be toggled after creation...)

Cheers,

--
Ben