Legal consent is not a rigid idea. Consent is consent. If I consent to something, then I consented. This is just the way it is. What legal consent does is it says that sometimes consent will not be recognized as consent under the law. In the case of a free market on law, what happens is this:
Lorraine and George have sex. Lorraine decides to sue Bill for rape (nonconsensual sex). The dispute is between the two of them. What Lorraine is arguing is that whatever consent she gave ought not be considered consent after the fact. In this particular case, Lorraine had been drugged by someone, and George knows this and gets drugged-Lorraine to consent to sex with him. Well, now that Lorraine is sober, she knows what has been done to her. She knows that someone drugged her and that she and George had sex. She knows that George knew that a crime was committed against her and George took advantage of it. Lorraine knows this. Unless she forgives George (entirely or in part), nothing George says is going to mean squat to her. George can tell her all he wants about how she “consented”, but the fact remains that Lorraine doesn’t really give a shit about that excuse.
I suggest you read What Law Is and A Praxeological Account of Law. The dispute between Lorraine and George is not going to resolve itself just because George says to Lorraine that she “consented”. She knows what George did to her.
In the case of Bill and the doctor, the only case Bill can take against the doctor is that the doctor performed a surgery that Bill didn’t consent to. If Bill wanted to live, then his dispute is about how much the doctor is charging him. But that’s not Bill’s case. He’s just not going to pay. The doctor is the one who is going to sue over the money. Then if Bill says that the doctor took advantage of his delirious state, that he wouldn’t have consented to that price had be been in a normal condition, then nothing the doctor says to Bill is going to change his mind. They will either reach a mutually beneficial agreement or they will remain in open conflict.
Over time, the results of these disputes becomes custom, and parties to the dispute can point to the custom. Bill can say, “Look, this surgery costs $25,000. That’s what I’ll pay him.” Lorraine can say, “Look, the custom is you don’t have sex with a drugged woman. Not only did George have sex with me while I was drugged, the bastard knew that I was drugged. George knew he was breaking the custom.”
“Voluntary slave” contracts would almost certainly be illegal in a free market in law system. Maybe there would be some societies that would allow for it, but the issue of consent would still resort to custom. Regardless, I consider “voluntary slavery” to be illegitimate. The only form of slavery that I even consider to have the possibility of being moral is enslaving previous illegitimate slave owners.
Customs, my friend, customs. There is no way of knowing for sure what people will consider to be legal consent in a free market in law. We can have our own personal opinions, but even then those miss the point. The point is that Lorraine has a dispute with George over what happened. That matter is not going to be settled until they both agree that it is settled. This is true under any system, even a statist system. It just so happens that people usually let the dispute end when the state says it’s over. But this is not always the case. You do see revenge killings or assaults or robberies every so often. Those people didn’t consider the dispute over when the state said so.