As far as I am aware, Dominated creates only get access to your At-Will Powers and these can be used against any creature, often an ally. Suggesting that you would never attack an ally expands to the you would never attack yourself.
The 4e definition of "your allies"
specifically excludes yourself.
In general, if a character
wanted to attack himself (for whatever mad reason), I can see absolutely no reason not to allow it. Likewise, if a character wants to allow another character to attack him, again I see no reason not to allow it. In fact, I'd consider any successful attack an automatic critical hit (as per coup de grace).
(This last is to handle the handful of abusive cases of "when you get attacked, you actually heal X hit points".)
In the case of dominate or similar powers, I would be inclined to
either allow a save to avoid it (as per forced movement into hazardous terrain)
or require a standard attack roll vs normal defense. Either the dominator overcomes the character's will to make him want to attack himself (save),
or the dominator forces the character to attack himself, but the battle of wills makes it an uncertain thing.
Or something like that, anyway. Also, be sure to check the specific power definition - some of them may explicitly call out whether the target can be made to attack himself or not.