These methods do succeed in changing the image displayed on a collapsed stack. Unfortunately, they also move the image to the top of the stack, which I am hoping to avoid.
These are stacks of 9 exposure bracketed images, repeated every 30 degrees around a pano. When I am ready to post-process, I will select each group of 9 and do an HDR process, then stitch the resulting images into a pano. By moving an image to the top and out of it's group of 9, I may end up an forgetting about that image (usually the best), and doing the HDR without it. But it doesn't look like there is a way of selecting a display image for a compressed stack without moving it to the top.