Your very rich post made me think of another bird -- well, birds -- from Rilke's First Duino Elegy (Stephen Mitchell translation): " Don't you know it yet? Fling the emptiness out of your arms/into the spaces we breathe; perhaps the birds/ will feel the expanded air with more passionate flying." Here is something much closer to what you call a "spontaneous, unpremeditated overflow of powerful feeling." But the world that Rilke and Hardy share is "our interpreted world" (also from the First Elegy: "already the knowing animals are aware/ that we are not really at home in/ our interpreted world."). Hence Pinsky's emphasis on the seeming of the second stanza -- Hardy's speaker is making the millenium hang over this scene, making the thrush mean something. I read the curious rhymes as calling attention to these uncomfortable acts of putting the whole thing together, making it mean something and never being certain that this made meaning, this interpreted world, is right, is really the world.
For surely that too is characteristic of modern poetry-- encountering things that writers know have already been encountered, every move taking place (really -- as your post indicates -- being placed, charted as if on a map) in space and time, but also in the midst of a density of earlier encounters. If I know a spot is "once-sacred," I also know that there is a story and language and music of its sacrality, and I am in some way incorporating that into my current experience of the spot. Rilke's beasts are onto something. I'm still not very comfortable here.
After reading your post, I googled you and found your blog, which I also enjoyed. Thanks.