I tried this with Zope 2.2.4. It works:
id=3D'entry_%d' % len(self.objectIds())
self.manage_addDTMLDocument(id, title=3D"", file=3Dcomments)=20
doc=3Dgetattr(self, id)
doc.manage_addProperty('guest_name', guest_name, 'string')
I hope you haven't given up yet.
Markus Kemmerling