Jesus is not an avatar of the Logos, a mask the Son assumes in a transient or extrinsic fashion, or a part he plays in some grand cosmic charade. When God becomes man, this is the man he becomes--and there can be no other. That is why it is silly to ask the questions that bad theologians, or casual catechists, or well-meaning Sunday School teachers have sometimes felt moved to ask: whether the Son might have been incarnate as someone else-as a Viking, or a Nigerian, or a woman, or simply another first-century Jew. The Logos, when he divests himself of his divine glory, is this man: between this finite historical individual and the eternal and infinite Son of God, there is no caesura. Jesus is not a manifestation of the Son, but the Son in his only true human form.
I certainly sympathize with Hart's general point, that the person of Jesus was not a triviality in the divine plan. There is no person of Jesus apart from the Incarnate Word (and, as far as both Jenson and Hart are concerned, no Word apart from its incarnation in Jesus). I even agree with the rejection of trivial musings like "could Jesus have been a Viking" or the more common "could he have been a she"--though this is more because I think they are unproductive than fundamentally unsound.
Where I find myself forced to dissent, however, is in tying every particular of Jesus to the eternal Son. Hart seems to insist that the Son could not have come as a different ethnicity or a different gender not for pragmatic reasons but for essential ones. This leads to other trivial (but I imagine damning) questions, like "if Jesus were an inch taller, would he cease to be divine" or "if he were balding" (instead of having a full head of flowing chestnut hair, as we all know he had) "would he no longer be the Son?" There is of course a sense in which it is true that the Son became incarnate as he did, the way he did, because that was the perfect time and the perfect manifestation for the perfect purpose of God. At the same time, to tie those contingent realities to the eternality of the Son seems to contradict Hart's normally strong rejection of any limitation or finitude in God.
Perhaps he wouldn't disagree with this (though there is nothing in the above quote to make me think he wouldn't), but I would think it safer to say that the Son could have come in any way he chose but, being perfectly obedient to the will of the Father, chose freely to come as he did, when he did, for the purpose he did. That does not, I think, mean that I believe Jesus was merely an "avatar of the Logos."