Here it quotes in the name of Kuntres Ish Itair 51, that it is dependent upon anatomy, which means a left handed person uses the right hand. He is talking about "Ashamnu" which only some Nusachos ever say with Tefillin on, so it isn't explicit that Tefillin doesn't change the consideration.
Here it specifically addresses Slach Lanu (#27) and says that the right hand is used. The top of the page indicates that most of the rulings there follow the p'sak of R' Chaim Kanievsky. T'fillin are not mentioned explicitly.
Although Tefillin are not explicitly mentioned it would be quite an oversight to not mention it, as that is almost always said (by men) with Tefillin on, and I have observed lefties doing just that.