Jens-
He can add the namespace using InitialiseSettings.php,
but this won't help
his case of special treatment of these pages except for not counting as
articles. No special handling in sorting, no list of portals, etc.
For some reason I was under the impression that the namespace text was
ignored during category sorting. That does not appear to be the case.
However, the namespace code *is* inserted into the CSS of every page, and
that also applies to extra namespaces, allowing for stylic.
Regarding the category pages, the correct solution to me seems to be this:
http://bugzilla.wikipedia.org/show_bug.cgi?id=450
Regarding the "list of all portals", a generic solution would probably be
best. Namespace filters would make sense on many pages - Special:Allpages,
Special:Newpages, Special:Recentchanges, Special:Most wanted ..
So, still some work for Aoineko to do if he wants to :-)
Regards,
Erik