Ken--OK. I had something like that in mind to keep from dropping the collar into the wall.
Thanks to your advice, I was able to rotate the white nylon collar piece 90 degrees clockwise, so that the larger notch was at the top. Then I was able to draw the valve assembly inward to within 1/4" or so of the shower wall, but it was still behind the wall, and I could feel that the polybute plumbing lines weren't flexing any more than that.
See, the opening in my shower wall to access the valve is much smaller than the opening shown in Johnny Air's photo, mine is 1 1/2" in diameter at most. The screw holes for the bezel plate are outboard of the hole in the wall. So I guess I'm back to chopping away at the shower wall. Ergh!