This works when you configure Google Photos to also show photos from Drive. However, now it seems impossible to exclude the photos from some folders from showing up in Google Photos (I've tried a .nomedia file).
Another way to achieve this would be through the special 'Google Photos' folder in Drive; except that it won't sync photos back to Google Photos unless you turn on the setting to show all photos from drive.
Does anyone know if what I'm trying to achieve is even possible? I'm curious to hear your experiences of the Drive/Photos combination!