Oma, your suggestion to expand the canvas is great - gave me an idea for a work-a-round to more precisely re-shape pictures.
1. After expanding the canvas all round the picture, I could duplicate the new layer 3 times.
2. Then, by selecting a different quarter of the picture in each of the 4 layers, I could eliminate everything else on that layer, and then apply the 'octagonal/quad reshappe/matte...' plugin on each remaining quarter of the picture.
3. In effect, I would then have created extra nubs with the plugin for the picture by working on a quarter of the big picture at a time.
4. Then, of course, by flattening the four layers, I would get the final product.
5. Still hopeful for a better solution.