User blog comment:Jenburton/An Update on Message Wall/@comment-3388044-20111001062627/@comment-91.48.244.194-20111001191758

I don't believe there's any limit to the number of namespaces, let alone such a low one. Wikia has a guideline of not allowing more than X custom namespaces per wiki, but that's more to avoid having to deal with constant namespace requests, I guess.

Bottom line: Creating a new namespace for this feature would of course be possible.