It's not impossible right now, it's just no one has done it yet. There'd need to be architecture changes certainly, and it'd be a lot of work. But that's kind of my point there, it's a lot of designer man-hours to put something together. And as of now, there's not even much theoretical evidence that it would significantly improve performance.
Its impossible, just edited my post to show you the theoretical side of it at least on the CPU side, nV and AMD don't really talk about how their caches work indepth to the point that we can contrast them, but they should work similar to this as we do have the theoretical latency hits of different level caches on GPU's at least for GCN it was in one of their white papers.