In 'Question Concerning Technology', Heidegger asks a question referring to his own comments concerning the Rhine now as a dammed power source, “the Rhine is still a river in the landscape is it not?” To which he answers “In no other way than as an object on call for inspection by a tour group there by the vacation industry”. Hmmm, slight exaggeration I think (especially as in the previous sentence he already states that the river is now primarily a power-source). Seems here that Heidegger is the one reducing things to mere present-at-hand.
 

For whom is the phenomena river ever other than present-at-hand? I supposed it's ready-to-hand for the ferryman who's crossed it so many times, but he's been made redundant by the new bridge that links the two river banks.
 

Wait I am slightly confused. I am by no means willing to give up rhetorical flourishing as a matter of arguing and disclosing truth. I am sure that if one were to go line by line of Being and Time we could play the 'gotcha' game on 'Heidegger and Rhetoric' but specifically when Heidegger writes that, "the human being is never present-at-hand" he is not exaggerating at all. By definition as you later mention the human's way of being is Existenz. Sure there is a phenomenological feeling of possession concerning the body, or realizing the body as a natural instrument of science and medical technologies but to say that the human being (which is not reducible to the body) is present-at-hand seems to be actually exaggerating his point, which is just to note the singularity of human being in disclosure. To say that the human first and foremost "Has" a body is to make the human-being a fundamentally disembodied Subject. In many ways humans are their bodies, yet I accept there is much to be discussed on this.
 

I don't know Heidegger's answer to this (I could speculate...), and I recently read soemone saying that Heidegger does have a Subject, but it's a void because everything is in the world.

Anyway, synchronistically I just stumbled across Kevin Aho's book Heidegger's Neglect of the Body. That might have some answers.