One of the “joys” of being a technical artist is solving unique problems. Problems that only occur at the nexus of a project’s platform, style, and design. I don’t think this is one of those instances, so I’m going to share my results.
This shader/script combo will let you add a flipbook texture to a Unity generated Sprite Atlas, and still animate the flipbook via the shader. The tricky bit was normalizing the UV information before animating, and then doing the reverse afterwards. There are a few other bits of setup info, but it is pretty straight forward.
Hope this can save somebody a few hours.