The Anxiety of Influence
For what it's worth, I think that this sort of thing is definitely legitimate. I like things that cross disciplinary boundaries. And I think that authorial intention (defining author broadly here to inclue all artists), although important, is not the only important thing out there. Don't get me wrong, the author is important. I don't think that the author is "dead." The reader's perspective on the text (defining text broadly here to include all art), bringing their entire perspective is also important and meaning is discovered in the interaction between the reader with all of their history, knowledge, experience, etc. and the author with all of their history, knowledge, experience, etc.
As to whether or not the painter had the Suffering Servant in his mind: Umberto Eco has a great essay called "Borges and My Anxiety of Influence" in On Literature in which he explores/outlines the concept of influence and all of the different dimensions of influence. He originally gave the essay as a talk at a conference in which scholars talked about his work. He talked about how intrigued he was by the literary echoes and influences that people were claiming his work contained. Some presenters he knew were right on, because he had read the work cited and had had that work on his mind when writing his piece. Some presenters he knew were wrong, because he had never read the work cited. Other presenters, he was skeptical at first, but then he remembered that he had read that work, but had forgotten about it. That work is still an influence, even if he wasn't directly thinking about it. It was in the back of his mind or had shaped him in ways that weren't direct, but definitely influential. The painter likely knew of the suffering servant concept at some level, even if he wasn't directly thinking about it. Eco also goes onto to say that the whole thing is more complicated than all that, because there are works that influence other works that we then are influenced by. At the very least, the suffering servant influenced the writers of the Gospels and the Church theology that influenced the painter.
Additional discussion and comments welcome...