A super secret namespace for the purposes of archiving forbidden knowledge or the like really isn't possible under Mediawiki, except for text articles only. Any media whatsoever, images, movies, files of any kind, are stored in the "media/image" namespace, which is accessible to everyone.
I could create another hidden namespace for such things, but any images, etc used in it would still be theoretically visible across the site.
If you really want a "Dark Archive" of some sort, Mediawiki is likely not going to like what you have planned. A second Mediawiki installation might be what you're looking for, perhaps with some interwiki linking. Apparently this is possible
, though I am highly dubious of the whole affair.
What exactly do you require? If it's just a scratchpad of some sort, I think another closed off namespace like FGC_Edit should do fine, apart from the images lurking about in public.
'fgceds currently include only myself and Reichu, although the FGC pages are visible to everyone, but not the FGC_Edit pages. Thus, only myself and Reichu should currently be able to view pages in the FGC_Edit namespace. Could someone check this. Additional FGC editors have not yet been assigned.