I've added __INDEX__ and __NOINDEX__ magic words in r37973. A configuration option might be added that would optionally disable them, or I might be reverted for other reasons, but the capability for user-added per-page indexing changes has now been written, in any event.
As I implemented it, any user can add or remove the indexing keywords. This is necessary given that they're implemented as magic words and not a special page or whatever. I did it that way because Brion seemed to favor it in his comments on bugs 8068 and 9415.
This would allow namespaces to be no-indexed by default (with a config change), with __INDEX__ used to re-index individual pages, like policy and essay pages in the Wikipedia namespace but not procedural pages like DRV or RFA.