The rules are mum on this, as far as I know, so we immediately enter house rule territory.
In my group, we had a more general discussion about whether you can hit a friend with an attack power to gain some beneficial effect. After a great deal of tought, I (as DM) decided to be generous.
* You can hit an ally with an attack power
* Each part of an attack power is optional, so you can hit for the special effect only, ignoring the damage part - just as you can ignore the knockback a power has if you don't want to use it.
The reason fort his is that my players feel their encounter attack powers are lacklustre; allowing many of them to be used like utility powers on friends gives them more interesting options.
We have not dealt with the idea of attacking yourself, but I suppose I would not allow that.