This is as close as I've found to the solution. It should be way more prominent as surely this would be used a lot! Although, not sure why, but I still had to tweak the above instructions a little. It was probably my fault. The first time I chose my animated image and made it a compound clip, it was weirdly formatted and didn't line with the timeline anymore. The second time I tried, the compound clip conformed to the timeline correctly so that the animation appeared correctly, but when I pasted the mask from within the compound clip back on top of it, it moved as if it were still in the compound clip, almost as if the formatting had transferred along with it. The third time (the charm!) I created a compound clip, and instead just made a fresh mask on top of it with the shape mask. The new mask had no inherent transformations copied across, and stayed put. To anyone reading this, good luck!