Sure - as the moderator who reopened the question, let me elaborate on what I believe the current consensus is. This is not a statement of site policy, but rather my take on how we currently do things.
I think that the current consensus is to accept questions on not just anime and manga, but also things that are part of the larger "otaku" industry/community, which I think uncontroversially includes (e.g.) light novels and visual novels, and perhaps-controversially includes things like Vocaloid; games in the style of KanColle and Girlfriend Beta; whatever Love Live was when it was first created (a "multimedia project", I guess?); and so forth.
I offer as evidence the following:
- We have lots of questions about visual novels without anime/manga counterparts.
- We have questions about Touhou, which is, at its core, a series of self-produced video games. Despite there having apparently been a few official manga (I don't really follow Touhou), most of the questions are not specifically about those things.
- We had a question about Kantai Collection that is more about the game than anything.
- We had a question about the Shuuen no Shiori song series, and I would be surprised if there were opposition to other questions about Vocaloid song series that lack anime/manga counterparts. (For example, if we had had a question about Kagerou Project back when it was still just a song series, I'm pretty confident we would have been fine with it.)
- We even had this recent question about Hatsune Miku herself.
- We have a fair number of questions about AMVs, which seem to me to be even one step further divorced from the core of the "otaku" community than things like Vocaloid.
The particular Miku question under discussion is, admittedly, going to turn out to have a silly answer ("no, Miku lacks any explicit characterization outside the domain of fanworks"), but I think it's still a legitimate question insofar as Hatsune Miku is clearly part of the anime-o-sphere.
Or, I guess what I'm trying to say is that the boundary between what is core enough to the anime industry as to be on-topic and what isn't lies somewhere beyond Vocaloid. (I can't really think of any topics that would be borderline, unfortunately, so I'm not sure how to articulate where the cutoff ought to be.)
(In closing, I would like to point out that we apparently don't object to questions about AtLA and RWBY, which aren't even peripherally anime-related.)