To request unlisted distribution for your app, send it for review as usual, then file a special form [1], and mention that in the review notes.
Source: I struggled with Guideline 4.2 when I tried to publish an app showing the bell schedule and other local information for the neighborhood school. Its audience is, indeed, not of Apple scale: the school parents living nearby. Apple refused it as 4.2 and only agreed to publish it as unlisted, which I was okay with, because sharing the link between the parents was not a big deal. Google had no problems with publishing the Android app normally though.
[1]: https://developer.apple.com/support/unlisted-app-distributio...