I have a custom namespace from which I want to move a few hundred pages, but it won't recognize the namespace's name as part of the page title, which I want to replace.
Topic on Extension talk:Replace Text
You can't use Replace Text to change pages' namespace, unfortunately - there's a bug about this here, although I don't know how to fix it. Are you trying to just get rid of a namespace, and move all those pages to the main namespace or something? If so, the MW script namespaceDupes.php could be helpful.
Oh well. No, I'm searching for a way to mass-move pages on a wiki that I have no shell access to. Their current namespace should remain as it contains pages I want to keep it for.