You mean so it shows a "-" in the page number fields instead of the page number?
In that case I think an attribute page@navbarnum which allows a value would make
sense, allowing to define exactly what is displayed there. That would allow for more
flexibility (e.g. you could have "iii", "iv", "v"... as numbering, which is used in some
books for the index or appendix).
If that's not what you mean, please clarify
