Really? I thought it stopped being valid once you threw and it was no longer holding the tsuka. Read that somewhere. In response to the OP, there was a match here where a one-handed player lost because his fake hand (in which he obviously has no feeling) touched the shinai of his opponent when it FELL OFF or something ridiculous like that. Was still considered a hansoku. Unless that arm is missing, I think it might need a kote on it. If you can still get hansoku from a fake hand, then the forearm should still be a valid datotsu.