I have tried UUID version 1, 3, 4 and 5, none of which Pages seems to recognize as a valid UUID.
I learned that 3 and 5 can accept a namespace. So am wondering if there's a specific namespace that needs to seed the UUID when it's generated. Apple documents it's own UUID, which perhaps I could generate with X-Code, but I'm not sure.
This is one of the Pages-generated UUID's:
9B7722F5-8029-4454-A8CA-AE6202BB5CA6 | my_shape