Unwrap mirrored object as if the mirror modifier were applied? Use different UV per side

Support Modeling
1

I have a character with shape keys but want to have an asymmetrical texture, but I can’t apply a mirror modifier with shape keys. How can I, if even possible, have Blender unwrap the object as if the modifier were applied, without actually applying it? I can “view modifier” in edit mode and select both sides but it still just unwraps one.

2

Enable “Flip U” in the modifier?

3

That may work, though I still can’t see the additional UV islands in the UV editor to line up the texture.