No part of the friends locked post becomes public; only its existence is revealed.

I disagree. Content is revealed even though it's only that the post contains a link. However, as you then go on to point out, this is already possible in some circumstances so the solution isn't to pretend it doesn't exist but, quite the opposite, give it as much publicity as possible (and adding it to dw as a feature would expedite that).

I'm biased though. pingback_bot is my new boyfriend. ;-)