I've recently re-read Target Liberty's criticism on Walter Block's hypothetical slavery contracts, and example #3 got me thinking:
Hypothetical Example #3:
A bailiff is working at a court in a big city downtown and the judge says to him during a recess break, "I'm hungry, go down town and get me a hot dog from the vendor?" The bailiff, under orders from the judge, says "yes sir, I'll be right back". The bailiff happens to be allergic and carries an epinephrine pen everywhere he goes in his back pocket. The bailiff gets to the vendor and stops and drops to the ground because anaphylaxis was caused by the walking (exercise) that he performed on the way down to the vendor. Three people are on the scene: the bailiff, the vendor, and a wealthy person. As the bailiff falls to the ground, the epinephrine pen falls out of the bailiff's back pocket. Both observers stand there and don't do anything, yet put two and two together that he needs his epinephrine shot. The vendor goes up to the bailiff and says "I'll administer this shot for you if you pay me 10 million dollars. The wealthy man says "I'll give the vendor 10 million if you be my slave for the rest of your life". The bailiff, having no other choice than to avoid the risk of death, nods his head in agreement to the terms of the wealthy man and the vendor. His life is saved, yet feels like there was no possible way to avoid the situation because he was faced with life and death. Would the court uphold the decision regarding the bailiff being a slave to the wealthy man for the rest of his life?
Usury - especially in the context of life-threatening distress - has always been frowned upon through most civilized societies, as has been withholding aid in such distress, and it has been enforced by law. In a pure AnCap society, nothing but their personal ethics would keep emergency doctors, EMTs or fire-fighters from extorting millions of dollar or slavery contracts out of anyone in critical distress - and purely economically, they should at every chance they get, as the price for their services will take a sharp spike upwards along with its demand for a very limited time window only. This is obviously not a very desireable course from the society's POV.
Let's assume these extortionate contracts explicitely rule out revision by private courts, and that there is neither a minarchist public court that could bring down Force on the usurer. What recourse is there to keep usury in emergency medicine in check?
If I were the bailiff's lawyer, and I was trying to convince an arbiter or private court that the agreement wasn't voluntary, my main argument would be that he was not of sound mind when he was in anaphylactic shock. People on the verge of dying are not always rational, especially if he was somewhat deprived of oxygen. If it were possible to be of sound mind while in anaphylactic shock, who's to say the bailiff would have agreed to be a slave? Maybe he would take his chances with administering the shot himself, or tragically just accept his death.
Most likely, an Ancap society would treat it the same was as if it were a mentally handicapped person or a child. That consent is not recognized as genuine, because one of the parties was not in the condition to be giving consent to that type of agreement.
It's also worth taking into account that as you said, most civilized people are against usury. You aren't going to get very many employees, investors, or suppliers, if the majority of the public hates you and what you do, especially if you have to compete with emergency medical services that are funded through means the public is more fond of, such as health insurance or donations.
Hope this answered your question.