In parallel with this, I'm wondering whether there is a case for having more than one "category-like" namespace, so that we can have a separate set of "categories" for project-related stuff. [...] It would have been nice if we could have had something like [[wikipedia-category:articles which survived deletion]] to keep this stuff in.
You know, I don't get why this was never done. Back when categories were first introduced, I expected this to be an obvious thing. I expected there to be at least eight new namespaces: 'Category', 'Image category', 'Wikipedia category' and 'User category', plus all their corresponding Talk namespaces. Now that we also have templates and the help namespace, there should also be a 'Template category' and a 'Help category' namespace, along with the relevant Talk namespaces. In fact, templates may have come before categories -- I forgot.
In fact, I would go even further and contend that images that are used solely by the Wikipedia namespace should be in a 'Wikipedia image' namespace which in turn should be categorisable by a 'Wikipedia image category' namespace, pages within which you can discuss in the 'Wikipedia image category talk' namespace.
The way I'm phrasing it probably sounds like it's getting really silly, but I mean it quite seriously.