@avandecreme I missed this question yesterday: "What [OpenSlide multilayer]support would do?"
OpenSlide only returns the central focus plane for any image format that has multiple focal planes, e.g., .ndpi. Ideally, it would learn how to handle multiple focal planes. This would include capturing and exposing the metadata about the various levels and adding a way to select a level.
There is a comment on the site indicating that they are in need of sample images with multiple focal planes, and I am in the process of getting some of those which can be freely redistributed.