Meh I'm not an ancap but I'll throw down a little anyway.
AnComs aren't really traditional anarchists AFAIK, though they are similar. Proudhon and Marx were competitors within socialist ideology.
Also, I'm not much for this sort of discussion. I generally perceive property as an observed phenomenon (so I guess this is what you mean by "just are" - property is what it is because people have and will continue to perceive it that way) that arose organically, and not something that can be implemented on any sort of large scale or permanence.
I guess it doesn't matter if you're only advocating a voluntary (small) AnCom (or AnCap) community.
But all previous attempts to redefine property on any large scale have failed and necessarily have to be implemented by force. Idealist marxist-leninists realized that the only way the would be able to implement a 'revolutionary' redefinition of property was through force.
And small voluntary societies dating back to Pythagoras always seem to die off after a few generations or less.
capitalist concepts of property
This just boils down to your definition of "capitalist concepts of property."
I define it as private property. Seems to me that animals and humans originally define property as the labor put into transforming natural resources into goods, and within the confines of the owner's ability to defend what they claim to own.
Not sure if this is correct, but I've always thought of it like this... One monkey pulls a fruit from a tree - that is his fruit. One monkey pulls 20 and hordes them, he will have 20 as long as he can keep them from being taken. Other monkeys naturally collect fruit - some choose to hoard and others don't. But all monkeys can agree they don't like having their fruit taken. They form a tribe and collectively recognize this standard under common interest. Taking fruit is now recognized as "stealing," but they are all still individually responsible for their own fruit. Two monkeys can share fruit if they want (make an communist society) - this is then a contract... Etc.
I think most of private property can be covered by this definition up to financial capital.
The corruption of my definition is that the state also forces me to pay for the protection of other people's property. A band of monkeys is now forcing me, under threat, to give up 1 fruit per week to feed militant monkeys to protect another monkey's fruit horde. His fruit horde has exploded uncontrollably as his protection is unfairly subsidized.
I think that monkey should use his own fruit stock to pay for the militant monkey protection of his fruit stock.
are only maintained because of state violence
Not necessarily. As I have illustrated above, they could be maintained by a private entity. Certainly a violent one though.
an individual's claims over land would only be as good as what the surrounding community thought was fair and reasonable
This is a moot point because all of society is defined in this way. If society suddenly up and collectively decided to go Mad Max, that is exactly what would happen today, state or not.
We all collectively recognize private property because we don't personally want our private property taken or violated.
If we take for example, large corporations in the third world. Even the most ardent capitalist surely must pause when considering whether the decision of workers to work in these corporations truly resembles anything like a "voluntary" decision.... Can we really say this was a free, voluntary choice? I find that difficult to accept. It seems that again, we have the state forcing workers to surrender the product of their labour to the capitalist
Large corporations in the third world generally petition the state for contracts. The state then contracts out work and resources. Taking oil in Nigeria for an example. AFAIK, the people living on the land are allowed to work for the companies. But the state is the only entity with the legal jurisdiction to sell the resources. If a man strikes oil on his village land, the state comes in and claims it to sell to large corporations. He can only personally sell it on a black market. Government officials are often majority shareholders in the profits created, too. It is basically the same with rubber and mining.
The problem in this situation is the state. This man has put in the labour and stuck oil - the state has come in and violated his homestead.