Interesting post which obviously has ramifications beyond food production.
With regard to Levinas, of course, interpretations are controversial. But I don't take the face to face encounter to be referring to geographical proximity. You don't have to be in the same location to have an authentic ethical relationship. I think this is the most plausible reading of Levinas because, if I recall correctly, he says this in one of his interviews, and also because, especially in his later work, he emphasizes language as the source of a face to face encounter.
As I understand it, "the face of the Other" refers to the vulnerability and particularity of the Other. And this can be expressed in language, not through the content (the Said) but through a dynamic, dialogic event (the Saying). I take it the key features here are (1) that the Saying invites response, whereas the Said does not–speaking to the Other rather than speaking about the Other, and (2) the Said articulates what has already been said, while The Saying is creative.
So it seems to me, Levinas doesn't give us any reason to think distant others properly attuned to the vulnerability of the Other could not have a genuine ethical relationship.
Perhaps we might want to say that genuine ethical relations are more accessible when they involve geographical proximity but it is not a necessary condition.
As to food production, you say "Pollan quotes a "post-industrialist" farmer who argues that, "the only meaningful guarantee of integrity is when buyers and sellers can look one another in the eye…".
Well, since when have there ever been guarantees? It is not as if there were no scoundrels "back in the day".
But Pollan is raising what I take to be the central moral problem of our time. We have dependency relationships with countless people we can never meet. With regard to industrial food production (or any industrial production for that matter) it is very easy to get through the day forgetting that you have customers for whom you are responsible. Thus, social structures have to be organized in such a way that such forgetting is not so easy. That is the main point of Reviving the Left.
What sort of organization might that be you ask? Well, that is not a conceptual question. My work is done here, he says evasively.
Thanks for the illuminating response! One issue unconnected to food that this discussion raises for me concerns the viability of virtual encounters. That is, is it possible for there to be an authentic (what a loaded word 'authentic' is!) encounter in a virtual environment (like Second Life or an MMORPG, or blog comments). I go back an forth on this, but if such encounters could occur in a virtual environment, this might provide a framework for the social structures Dwight is talking about.