Not really
Painless is governed by two things: whether an action causes pain, and whether there's a neurological structure that can receive pain impulses.
If the pain receptors are gone, something that would ordinarily be painful is not. So, in order to make your analogy complete, the prisoner's cerebral cortex would need to be removed. Only then are the two situations equivalent.
Also, since we are already beating people in our custody to death, it's kind-of obvious that the concern about pain has not been passed on to all our interrogators anyway, so why don't we work on that first?