See the object “Cube.GN_literal_unwrap” (I haven’t deleted it accidently, did I)? It should still have a GeoNode mod attached.
It’s a copy of the Cube with the original UVmap, split at seams, and “unwrapped” (points positions are set to UVmap coordinates).
Oh, and since I don’t know how to find seams in GeoNodes, all the seams have Crease set to “1” in edit mode (because we can access that attribute at least).
This GN modifier is then Applied and Shapekeys can be added.
Ok I didn’t realize that object also had geonodes. This is breaking my brain a little bit but I think I get it. I’m going to try and actually use it on a more complex object later (i.e. a head). Thank you again so much! I think this is- while not what I was planning on doing- going to work very well anyway 
That’s indeed useful method when you need more than 3-5 UV shapekeys and do not want to copy-paste nodes for hours. For access to shapes you can use Drivers. In fact, drivers are no-brainer when you work with complex animations which involve Armatures, shape keys, modifiers and shader effects at the same time. You will go insane if all of this isnt wired to helper bones in a single armature