I am not the OP, but I'm very much interested in how referencing works with ConTeXt so let me add to this.

You can write your own style, or your own modified style.

Is the publications manual still up to date? Is there more recent information available? How would I start writing my own style?

Concerning your two requirements for new styles:

(2) there is a clear, defined standard that can be followed.

This is actually the easier question: A few standard styles immediately come to mind: MLA, Chicago 16th and 17 edition in its various variants (author-date, note-bibliography, fullnote-bibliography, both note styles with and without ibid.), Modern Humanities Research Association... I guess having a model for all major variants (authordate, authoryear, numeric, alphanumeric) would be a good starting point.

Also, we can implement more styles if (1) there is a need

This is more tricky. I do very much like what ConTeXt has to offer for typesetting finished works, but I currently would not use it for writing a larger work in the Humanities. For smaller pieces I would probably write in Markdown, use pandoc to produce a context source file, and pandoc will me automatic citations via pandoc-citeproc (yet with a few glitches). For a longer work I currently don't have an alternative to biblatex. And I guess that is the point: Those with rather complex requirements for citations will either do them manually or use biblatex for this so it's not so easy to say if "there is a need". Having said that, I would really like being able to use ConTeXt here as well. So having a author-title style for the footnotes and the bibliographies would be more than welcome.