The workaround I use to solve this is to use Sections. Under "Post grades", it is possible to publish grades ONLY to students in a specific section. Since specific students can be added to a section, this is a way of limiting the visibility to a single student or a sub-group.

It is an inelegant solution, and Sections management in Canvas is awful to work with, but if you don't do this too often, it's a functional workaround.