Went for a third viewing at one of those theater/restaurants. Wow, Imax 3D really is a MUCH better movie-viewing experience.
So I picked up two small things I hadn't noticed before, which though small would lend themselves to the Banner-is-Loki theory.
Right at the start of the movie, when Thor is in post beat-down by Thanos... Loki steps over to Thor and says something along the lines of, "I promise you our star will rise again, brother". Given how hopeless their situation at that moment, I think it easily could have been Loki trying to give Thor some sign that Loki had a plan.
Second, when it came down to extracting the Mind Stone from Vision, you'd think having been part of hooking it up in the first place, Banner could have contributed something there. But he just completely shut down the notion of him being able to do it. Which if it was Loki would again make sense.
Like I said, small things, still don't know that I expect the theory to be right, but figured I'd mention them.