[Zope] Defining my own security

Okay, here goes... I have created a Zope Product that extends a Zope Folder object and I want to add a non-traditional security check before allowing users to "View" stuff in the "folder". For reasons that are too complicated to explain here, I can't use Zope roles and permissions for this check (in fact, this check should be done in addition to Zope's existing security mechanism checks). What I want to do is verify that a particular variable in the "Session" matches a specific property of the "folder". If it does not match, I want to raise an unauthorized error. Is this kind of thing possible? Any help will be appreciated. Thanks, Steve