With the landing of ⚙ D17093 Shader Nodes: Use layers from evaluated mesh shaders can now use uvmaps created in geonodes, you no longer need to use an uvmap existing on the base object.
You still can’t output uvmaps via a group output though, because that will still force it to a 3d vector and thus make it unavailable to the UVMap node.
Are the new uvs exportable with alembic, fbx, obj etc?
Yes. Any FLOAT2 attribute on face corners is now considered an UV map.
If any exporter doesn’t see them that’s a bug and I’d love to hear it before 3.5 is released
You just need to use ‘Store Named Attribute’ to save them, and not the modifier outputs panel, because the latter will silently convert it to FLOAT3 because FLOAT2 sockets don’t (yet) exist.