If you don't mind the terrain looping, I think it's possible.
Step 1: create a more accurate map of the terrain in the terrain graphics tag. Replace all '*'s for hexes that are not part of the hole with '.'s. This should keep the engine from drawing on the non-tile spaces. If this doesn't work, copying out all the terrain graphics base tags for the surrounding terrain and raising them to a layer higher than the hole image will work. I suggest the first option just to keep things simpler.
Step 2: make a really big image with the terrain you want. For simplicity, copy a section the width of the hole from the left side to the right. The image should then look something like this:
Step 3: use image.png~CROP(offset,0,hole width,image height) for varying offsets in pixels from 0 to just before the start of the repeated section
Should look like
The effect should be continual looping motion over the background.
For extra fun, do the clouds as a separate layer/image and have it move at a higher speed (i.e. fewer frames in the loop).
Edit: Looking at your WML further, it looks like you intend it to be walkable. The simplest way to add that would be to add a custom transition border that doesn't animate and is drawn on top of the animated backdrop with another image tag.
The reason I noted for simplicity in step 2 is that you could use ~BLIT and a second canvas image to show all the pieces if you want to minimize file size. It's much more complicated though.