Brian Magee on free will (Confessions of a Philosopher, Random House  p. 152):
I am entirely confident that if you subjected any determinist who is not a psychopath, however amoral his life, to outrageous and cruel ill-treatment, he would become indignant with you and protest that you ought not to treat him in that way. Ought and ought not would spring to life for him then, and he would insist on attributing to you the ability to behave otherwise.
Scotus on contingency (Reportatio IA prol. q. iii art. i; in Philosophical Writings, Wolter, tr., Hackett  p. 9):
And so too, those who deny that some being is contingent should be exposed to torments until they concede that it is possible for them not to be tormented.
And, of course Scotus is adapting Avicenna on noncontradiction (ibid., with a minor change):
Those who deny a first principle should be beaten or exposed to fire until they concede that to burn and not to burn, or to be beaten and not to be beaten, are not [the same].
What makes these legitimate arguments is that in each case the punitive middle term is doing real argumentative work: it is relevant to the subject at hand, and it does link to the conclusion by giving a reason to think that the person holding a position (that there is no free will, that nothing is contingent, or that to be and not to be can be the same) is claiming something they would not consistently be able to uphold, and, perhaps, accept only verbally. Any experience, in principle, would do; the reason for going to the stick is that it's an extreme case, where it's difficult to deny that you'd accept the principle (without sounding implausible). In none of these cases is it demonstrative; on its own it just shows the difficulty of consistently holding some claims, nothing more.
Have you come across any other instances of argumentum ad baculum that are legitimate, i.e., that appeal to what one would be able to hold under torture or beating, that do so in a way that is actually relevant, and that aren't merely threats, but arguments that a position cannot be held consistently through the full range of human experience?