Hello Scott,
Sorry about the delayed response. I was back in OH for a wedding.
That's fine, we can use Merriam's definition of the word 'slave'. Under this definition, the house elves of Harry Potter's world would indeed be slaves.
Using this definition, I would argue that slavery is not necessarily evil (though it usual is). If one wants to be a slave, then one should be able to be one, as long as that person can leave slavery if their change their mind. Of course, if one can actually exit the service of another, is one truly owned? If not owned, again, slavery would not be the right term.
Indeed, I think I would argue that ownership implies subjugation of the "property's" will. In which case, you are correct; ownership of thinking thing is wrong. I guess it is the 'ownership' aspect of slavery I object to, not the 'service without pay' aspect. If an artificial intelligence is built so that it wants to serve without pay, I see no moral objection to allowing it to do so.
My limited understanding of Stockholm Syndrome is that it involves abuse, which slaves should be protected from, morally speaking.
Is moderate corporal punishment okay? Applying the standard I set in my previous comment, I would ask, "Is moderate corporal punishment for children okay?" This is a touchy subject, but my answer would be "depends on the offense". Others object to all forms of corporal punishment.

Merriam defines slave thusly:
1 : a person held in servitude as the chattel of another
2 : one that is completely subservient to a dominating influence
Neither of those definitions depends on the matter of will. If you're held in subservience to another person, if that person "owns" you then you're a slave whether you want to be there or not. A person can choose to be a slave. It happened often enough in our past, called indentured servitude. I believe that that is just as bad. No human, even given the choice, should be in bondage to another. The second definition could be a case where one person is dominating another with their will. The one dominated could come to believe that the domination is good. This happens in Stockholm Syndrome. While that's not "slavery" per se, at least not how most think about it, it's not good.
I think if we get to the point of developing intelligences, biological or artificial/mechanical, it would be wrong to "own" them, but it will happen. There will be folk against it, but it will still always be a temptation for man.
So moderate corporal punishment, would that be okay?

I guess it depends on how we are defining 'slave'. I understand 'slave' as someone who, against their own will, is completely subservient to another's will. Thus if one wants to work for no pay then one is not a slave. Their will is not subservient, but inline with, the master. If one has a will that cannot be expressed because it is trumped another's will, I'm sure they know it.
A common understanding of 'abuse' suffices in this instance. If you'd consider it abuse if the victim were a child, it's abuse of the 'slave' as well.

And you'd also have to define "abuse".

So you're not a slave if you don't know you're a slave?

Going back to our debate, it all depends on whether suffering results from subjugation of the slave class. If we genetically engineer a slave race and manage to somehow ensure that they want to serve. Luckily, recent fiction has described just such a scenario. I'm referring to the house elves in Rowling's Harry Potter books. What we'd have to ensure if we were to create a 'house elf' sub-breed is that any radicals like Dobby have the means to exit service if they so choose. Also protections against abuse would be necessary.