I'm rolling around the idea of protectionism and justice in the free market. (Thank you for the extra links, btw. Great way to add value to your post and keep us thinking.)

I could see this working in an HOA (Home Owner's Association). People voluntarily sign an agreement, when they buy a home in the HOA's boundaries, to pay the HOA a yearly fee, and in return they get certain benefits. One benefit might be that the community contracts with a local helicopter flight service where if one of the community members suffers a serious injury, the pilot will immediately fly over and airlift the patient to the hospital. (I saw a community doing this recently.) One might be to have a private security force patrol the community.

So let's say you're not in an HOA community. You're just in a neighborhood with about 150 homes, and there's a movement that the owners would like to get together, pool some money, and hire a security force.
What happens with the homes that don't want to participate, for whatever reason? (Perhaps they don't like the idea, perhaps they simply can't afford to, etc.)
I guess the ones who were working together to hire the security force could hammer out details on that with the force itself. Perhaps if the force seems something hinky going on at a non-participating home, they don't take action.

Either way, the non-participating owners are still going to get the benefit of having regular patrols and therefore reduced crime in their neighborhood, even though they aren't paying.

I can see this rankling the members who are paying ("I'm having to pay for their safety!") and as the old lessons go, if one person isn't pulling their weight but still gets the benefits, others stop pulling their weight, too. The system falls apart. (We saw this in the first colonies here in [what became] the US.)

I realize that the de-facto response to a hypothetical like this is always "Well how is what we have now any better?" and/or "The current system of state violence is a much worse system than your hypothetical", but the fact is that we live in a society where fully half of the households receive monthly financial support from the government. They're vested in the system because they're dependent on it, so you'll never get them to change their minds. In short, until the economy collapses, the government implodes, or something takes out the power grid and knocks us back to the 1700s, we're stuck with this. Rather than simply saying "well what we have now isn't working/is more violent", we have ample time available to explore hypotheticals like this and come up with some reasonable answers.

Who knows, maybe we'll save some communities some precious time in the future, when they find themselves in a voluntaryist community and come across this issue.