[Grimburger]

Then you should hide them behind a url that isn't linked elsewhere on your site that you can easily propagate by word of mouth only.

    example.com/correcthorsebatterystaple

If you consider "word of mouth" to be public posts on a forum which millions can read at any time then block googlebot IP's

[jaystraw]

...after decades of what i considered friendship, here you are on hn talking about my horse battery staple

[tympious]

Yes, sorry, it was a rhetorical question in response to previous.

Taking either step you suggest (along with robots.txt or eqiv.), it would seem fair to expect that Brave, Bing, whomever, would not feel it their neutral/natural domain to include in a public index.