From #1465 (comment)

Apologies late to this ticket, am very interested in making use of this feature, but it would really need the full region to be created for compliance with viewers (see https://iiif.io/api/image/3.0/compliance/#3-image-parameters). Good examples from Tom Crane to illustrate this here: https://tomcrane.github.io/scratch/osd/iiif-sizes.html