I am with you in disagreeing with Eirenicon's assertion that self drowning in a bucket is impossible with probability 1, though I believe with high probability that it is difficult beyond the ability of most people. I was mainly objecting to your assertion that this couldn't be studied.

Also, merely holding your breath is not dangerous. You would pass out before suffering any permanent damage, and breathe normally while unconscious. It would be dangerous in an environment, such as under water, in which you could breathe normally after passing out.