[me:]In anarchy, some agencies will, and some won’t, have laws allowing compelled testimony. This strikes me as the best possible solution.
How is that supposed to work? If you don’t want to testify, I can shop around for an “agency” that will lock you up until you do? Hey, maybe if I pay them enough they’ll even torture you.
Rules of evidence have to constrain third parties. You can’t create an Autonomy Zone around yourself and demand that nobody make you a witness to anything without your consent. And without testimony (not necessarily compelled testimony), nobody knows who’s committing “aggression or initiation of coercion” and who’s not.
First things first: the standard disclaimer about anarchy. Anarchy is a system that is defined by what there isn't -- a state -- not by some specification of the society that will result. Thus, any notion I propound, although it's probably a good guess, is just that: a guess. Anarchy is what anarchy does, and I may be wrong.
OK, so how does it work? Well, first let's posit that coerced testimony is useful from the POV of achieving high-quality justice. (If it isn't, then there won't be much coerced testimony in anarchy, because the market won't produce many worthless services.) Given that it is helpful, then I think most people would agree to allow themselves to be coerced in order to be member of an agency that coerces. Of course, this is not coercion in its bad sense, since it is agreed to. It's just a contract, where the party agrees to testify freely and truthfully, with coercive sanctions if he fails to do so.
So most people will end up being governed by an agency which has coercive testimony, at least for internal disputes (between two customers). It is very likely that the agencies will extend that to disputes between customers of different agencies (which both allow coercion), since the same utility applies.
What of the minority that doesn't want to be forced to testify? If there are enough of them, and/or they have enough wealth, they can set up their own agency. Perhaps the rules will allow coercion within, but not without. Or perhaps no coercion at all. In either case, there is a mismatch with outside agencies.
Now, it might happen that in any given dispute, a person who can't be forced may agree to testify anyway. But of course, that doesn't solve all disputes.
In others, a person (let's call him Joe) will refuse to testify (and his agency will back him), and a foreign agency will want him to testify.
The first thing to point out here is that if the second agency is libertarian, it will not coerce Joe because to do so would violate his rights. But remember that this is not a libertarian minarchy, it is anarchy. The agency is not limited by libertarian ideas, and may threaten to coerce in spite of it being aggression. So what next? It threatens to coerce, and the second agency will defend its client. This turns into the standard agency conflict scenario. I've discussed it long ago (here), so read that.
The upshot of agency conflict is, typically, that they can foresee it, and if it is likely at all, they'll have negotiated it out long ago. War is incredibly costly, and bad for business; a last resort. In a case like this one, where a minority wants something that is reasonable, the likely outcome is the minority agency pays the other agencies, and they agree to apply the minority rule in conflicts. Thus, Joe ends up paying more to his agency (so it can pay others), but his right to not testify is upheld.
Or, it may be that the right not to testify is sufficiently unpopular that the equilibrium ends up with the minority being bent to the will of the majority. Joe will have to testify. But note that in this case, it's likely that his right is "bought out" by the agencies that want it. Also note that Joe still doesn't have to testify against other customers of agencies which don't coerce. This may seem a small thing, but it would allow a minority to effectively achieve their goals, at the cost of cutting themselves off from the majority (so that conflicts don't happen).
So, getting back the my interlocutor's questions. Yes, you might be able to shop for a venue to force Joe to testify, or you might not. Depends. Either way, I don't see a huge problem.
There is nothing in anarchy that defines there as being no official torture. However, I predict there won't be any, because it is not very productive and it is anathema to all decent people. The tiny minority who would want it is like the minority that want to murder freely. If they did start and agency that allowed it, all the other agencies would band together and annihilate them.
As for an "autonomy zone", well, that's exactly what you can do. Judith Miller is doing it. Anyone can. Because you control your tongue. I see no reason whatsoever that any agreement between third parties constrains me. If they want my agreement, let them negotiate with me (or my proxy).