It looks like all that's really needed on the scopes side is a documentation update. I've documented a "art-fallback" in the "components" dictionary of the category renderer. That allows a scope to specify a category-specific fallback image. I think this should be enough of a hook, but the shell will need to actually use the field if it cannot retrieve the artwork for a particular result. If no fallback image is specified, or the URI for the fallback image cannot be resolved, I guess the shell should display some universal placeholder image, so there is at least something on screen, instead of nothing at all.
It looks like all that's really needed on the scopes side is a documentation update. I've documented a "art-fallback" in the "components" dictionary of the category renderer. That allows a scope to specify a category-specific fallback image. I think this should be enough of a hook, but the shell will need to actually use the field if it cannot retrieve the artwork for a particular result. If no fallback image is specified, or the URI for the fallback image cannot be resolved, I guess the shell should display some universal placeholder image, so there is at least something on screen, instead of nothing at all.