Actually its fairly trivial to implement, especially for an initial pass.
Add new "Edit OP" and "Edit Posts" permissions, allow it to be a assigned to groups on a category.
That way, for example, all "trust level 2" can have Edit OP on Howto.
It is not a massive change and we do mean to get to it at some point.
There is also the idea of flagging a post as wiki somehow, but that would complicate the UI.