Notwithstanding NASA's policy on lifetime astronaut exposure to radiation, if they can reduce exposure on a trip to Mars below a lethal dose, I don't think they'd have a problem finding people who were willing to sign a waiver on that policy.
I think as it stands there's probably people who'd either sign it over and take the chance on a there-and-back trip and potentially get cancer, or view it as a one way proposition. Not sure whether the ethics of sending people to their potential doom would fly mind.
Remember that these people will get rigorous medical screenings throughout their lives following the mission. Even if they do develop cancer later on, it would be found early enough to be treatable in most cases. With the added screenings, an astronaut may benefit from a lower mortality altogether (than a normal person exposed to background radiation back on earth).
Kind of, there's significant risks involved in shooting someone into space but not the risk of a long lingering death from radiation exposure. I think that's where the ethics committees get a bit hesitant.
I fully understand someone wanting to do it despite the risks, if its their choice then they've made their decision.