Is it on-topic to ask questions about wiki writing in general? Wikis can be great resources but - in my experience - need special writing skills. Finding good titles, organizing content into pages, linking, etc. I found that the wikipedia and websites tags exist but there are no [wiki] or [wiki-writing] tags. I read that Wikipedia-related questions are on-topic but I know that other (project/community) wikis get into trouble precisely because they try to imitate peculiarities of the Wikipedia "role model" and (have to) fail at it. So I would expect that most Wikipedia questions and answers will be difficult to generalize (well, in future, I know there are currently only 5).
Just like everyone won't write fiction, not everyone will write wiki articles. I think this is completely fine, even if the audience of people who will find it helpful is limited compared to other types of writing. If the questions is only really able to help one person(like critique requests) then it gets closed because it can't really help others. Even if wiki writing only helps a dozen or so, it still helps people and should be able to stay open.
So yes, I'd say wiki writing is on-topic for Writing SE. Although you may want to make a tag for wiki writing if you plan on asking several questions on it.
Since wiki writing isn't done by everyone, it might be harder to get answers on them, but you seem to have accepted this already.
Note; I in no way have any idea if this is correct(I couldn't find anything saying 'no wiki writing content'), this is just what I feel is correct based off of existing rules & regs. You'll probably want to check with a moderator for a definite yes or no.