In Okinawan, what is the ン in ウチナーンチュ?
User ithisa conjectured that this ン is from ぬ, from の. Let's explore.
Japanese 日本人【にほんじん】 doesn't seem to have an equivalent [to Okinawan ン].
You're correct, but note that the Japanese on'yomi term 日本人【にほんじん】 follows Chinese grammar, where "a person from XYZ place" can be expressed as
person. For a proper Japanese construction, you'd have to compare a wago phrase like 大和【やまと】の人【ひと】, where we do indeed find an equivalent with that の.
Could it be a reduced form of Okinawan ヌ, the equivalent of standard Japanese の?
Yes, it is!
The Shuri-Naha Dialect Dictionary is one very useful online source for readers of Japanese who are interested in "standard" Okinawan.
The site has easy lookup starting from either a Japanese word (click on the appropriate starting hiragana) or an Okinawan word (click on the appropriate starting katakana).
For instance, we find that this site has an entry for ンチュ, which explains that this means の人, and is a shift or abbreviation from fuller form ヌッチュ. We can also peruse their entries for possessive particle ヌ, equivalent to Japanese の, and for noun ッチュ, equivalent to Japanese 人【ひと】.
(FWIW, there's also JLect.com. I've found their coverage to be a bit spottier for some things, and the site is not as user-friendly. That said, they do include resources for Okinawan, as well as other varieties of Ryūkyūan like Amami or Miyako, which can be super useful.)
Please comment if the above does not fully address your question.