To the topic,however:

I think that it is absolutely right for you and some of those you refer to to point out that the "harm principle" implicitly incorporates a prior understanding of legitimate property rights. And there's the rub. Many libertarians have a very primitive and naive understanding of property rights - both in dynamic and static terms.

The Lockean fairy tale about abundant gifts of nature and some prehistorical first claimants tells us virtually nothing at all about what current property right titles are legitimate, and the assertion that "one has the right to his property" tells us virtually nothing about what that property is or isn't, can be or can't be.

In some societies, for instance, land can't be owned but can only be possessed while being actively used. In some societies land can't be farmed, but can only be "used" to hunt and gather natural fruits. In some societies cows are sacred, cannot be owned, and can wander hither and yon inflicting damage and havoc at their will. Those aren't "interferences by government" or "impositions on individual rights" they are parts of the INITIAL DEFINITIONS of what constitutes some of those rights. There is no discretionary government action involved at all.

And that last sentence, perhaps, isolates an element of this sort of discussion that you need to make more explicit in your discussion of a "harm principle." One can only be harmed with relationship to one's LEGITIMATE expectations. It is no harm to me if I can't demand of you your automobile because it would be convenient for me to use it today. Similarly, I am, perhaps, not harmed if I don't have "a right" to torture puppies. Do the puppies "have rights?" Ah, no, but neither do I "have a right" to inflict unnecessary harm upon them IF that is the way that things are set up "to start with," and not some bureaucrat's or policeperson's discretionary decision down the road.

To put that one final way: every society starts out with a more or less detailed definition of what can be owned and what can be done with what is owned, a conception that usually is then "tweaked" here and there at the margins over time. That isn't "intervention" or "unlibertarian," it is a recognition of the existence of the economic fact of universal scarcity and the establishment of a "property rights regime" to systematically address that scarcity.

how did a Southern slave owner justify his definition of property only a few decades ago? i mean - they were convinced that they had all the RIGHTS in the world to use and abuse... BLACK RIGHTS - how ludicrous... we feed our slaves and protect them from wild animals and such... they went to war with their fellow humans over it. Why the North went to war is clear - they valued the economy (ecology) advantages and considered blacks to have the right to freedom.. but the South considered blacks as beasts - they went to war with fellow humans in order to defend property and their way of life.

why has the US constitution suddenly changed in the 1960s - taking out "blacks are 2/3s of a human"?

do we believe in the benefits of free will and free economies or do we believe in oppression, tyranny and plan-economics... and if this holds true for the economy why should it be different for ecology and all other emotional, intelligent beings besides ourselves.. yes even women?! why should a white male benefit from the "liberation" of black women? because he is a "liber..."

the question is NOT - can one be a "liber.." AND for the liberty of animals... the question that needs so desperately to be answered is... "Can one be a libertarian and NOT support a free and self-governing animal people and ecology?" Free markets, free agents, free...

The south was simply too afraid and stuck in the a dangerous world-view regarding what makes people happy, what makes them rich and what makes them civilized. They were, despite some nice houses and fancy dresses of few, barbarians in many respects.

The contract theory enunciated above would seem to limit the reach of the power of the state to those who could consent to the power of the state. (the state is legitimate because I consent to it; in some sense, it's my knowing consent that protects me). That forgets that the contract is a fiction: I no more consent to vest the power of coercion in the state than a monkey does.

Your "slippery slope towards justifying any law based solely on societal norms" reflects the reality that psychic pain suffered by members of society at large is sometimes a legitimate basis for law. The proper balance between the claims of the many who are suffering psychic pain and the claim of the individual who desires the liberty to do as he pleases is not marked by the bright line extreme libertarians profess to see.

Put another way, there is no mechanistic algorithm for arriving at just and wise policy. That Objectivists insist on believing otherwise seems like a good basis for the perjorative "Randroids."

If you saw a dog that you knew to be ownerless being abused by a sadistic stranger, could you justify intervening? Most people could. Now what if that dog belonged to the stranger? I think most people could still justify intervention. Perhaps ownership of animals is predicated on effective stewardship. Even a libertarian would object to the abuse of a resource that doesn't belong to the abuser.