I'm not hugely surprised that this is possible.
However, I suspect a lot of stale milk gets reused in other products, though I don't have links for you, and I don't know if the usual industrial collectors could use soured milk. The proteins in it are valuable, even if they're just dried out and tossed in animal feed.
That term "Victorian milk paint" is, actually, literal. It's similar to using the proteins from eggs for tempera painting.