If I'm understanding you correctly, your position is that since downvoting has been corrupted we need a new egalitarian mechanism, such as calling out jerks, and that if we all use that mechanism reliably we can clean up the community.

Well, not exactly. I'm not proposing "calling out jerks" as an alternative to downvoting as a mechanism for weeding out low-quality contributions. I'm saying that there are different kinds of contributions to the community that we want to discourage. We want to discourage poor-quality comments, of course, and I still think downvoting is a decent (not perfect, but decent) way of doing that. I don't think the block-downvoting we've seen so far changes that.

But we also want to discourage harmful contributions that don't come in the form of poor comments. Passive-aggressive voting behavior is a harmful contribution to this community, in so far as it jeopardizes the "community" aspect. Voluntary communities should, on balance, be pleasant places to be in, at least for the kind of people the community wants. Block-downvoting makes the community a less pleasant place (to the extent, apparently, that it has already de facto driven out one valuable contributor) without any significant countervailing benefit.

The karma mechanism was not designed to prevent this kind of harmful contribution, and I was saying that in the absence of a formal method to discourage it, calling out particularly egregious offenders might be a reasonable strategy. Of course, it remains to be seen whether it will have any impact, but I think there's a non-negligible chance that it will. I would much prefer seeing one of the formal mechanisms proposed on this thread being implemented, but I think the chance of this happening in the near future is small.

Which leads me to ask: once we establish that convention, and rogue agents therefore start (incorrectly) calling out valuable contributors for being jerks, what ought we do then?

I think there are a number of signs we can use to ascertain the credibility of a call-out. If a call-out doesn't appear credible, down-vote it and explain why you don't find it credible. If an agent is making a habit out of calling out people, down-vote him/her and perhaps express your displeasure. If calling out becomes undesirably common, start discouraging the behavior in general, without regard to the credibility of the call-out.