In some ways I'm sympathetic to this thesis, insofar as it emphasizes and keeps open the gap between the content of the art and the effects it has; or, similarly, because it doesn't act as if the artist's "intent" has anything to do with the artwork's reception and utilization in the world. I found Walsh's article maddening in part because he wants to use film "to revive protest, anger, and outrage, the desire to see the world changed from top to bottom." I don't have a problem with this per se, but just because films are manifestly political, or angry or whatever, doesn't mean the responses to them will be a desirable, beneficial, or even political one at all. The notion that you can calculate and modulate the response to a work of art before it's even produced, let alone distributed, assumes a lot about the politics of the subjects it's trying to reach. I'm tempted to say it's the left-wing political version of Hollywood's filmmaking by focus-group model. But I think it might be worse: Hollywood at least bends to the desires of consumers; Walsh assumes you can just manipulate them any way you want.
My problem with your thesis is that it seems tremendously utilitarian, and perhaps simply does the opposite of what Walsh does: assumes the artwork to be completely empty of politics, a cipher, or at best (as in your example of The Birth of a Nation) as having a negative, inversive relation to politics. Somewhere Badiou says something to the effect that aesthetics needs to find the "meaning" of art outside the actual work of art. I can only read this as reactive, a revulsion at the commodity form. Both Badiou and your tentative thesis would seem to erase the existence to say nothing of the independence of the artwork. I think it's essential to insist on this independence while also trying to gauge its effects.
Note that I'm saying independence and not autonomy; the latter is a sort of bourgeois conception of art, as transcending the social and political. At first blush, though, your theory (like Badiou's desire to find the truth of art outside the object) seems to do something very similar: it forgets that the artwork itself is produced by subjects--almost always collective subjects--living in a certain social formation at a certain time. Which is to say, even if it creator doesn't try to, the artwork reflects its time and is political.