Sadly, it doesn't seem so. The fact that there is no clear place to do this is honestly quite ironic given that Apple requires all third-party apps to have an easy way to report rulebreaking content…
There is actually! I was told about it over the phone with an App Review rep a few weeks ago. I didn't write it down unfortunately, but I think it was appreview@apple.com - googling for that e-mail address returns some results, so I am pretty positive that was it.
Awesome! It seems that most of the references in search results are around interactions between Apple and developers. But it definitely looks like the best place to send violations as well. Thank you!
I just sent them an email. Other developers should too because everyone should play by the same rules.