[Zope-CMF] Permissions in Document class

[Jeff Ross]

I am using a CMF Site with the NFGNav product, and would like to have a
checkbox on my document_edit_form that lets me determine whether or not
items show up in the navigation. This occurs by assigning a permission,
"Reference in NFGnav" to objects.

The problem is, this permission doesn't show up in Document objects. There
is a very limited set of permissions that show up for document objects and
I'm wondering why. How can I get "Reference in NFGnav" to show up in the
Document object?

Do I have to subclass it?

Jeff