What you could do is, that you have lowres model, then add multiresolution modifier after that shrinkwrap modifier. after that you subdivide the multiresolution modifier until you get the details back. then apply the shrinkwrap modifier and the detail is then saved on the subdivision level you were using on the multires modifier.
after that you can unwrap the base (or actually do that first). thing with multires is that its something to use after your pretty much done with the actual sculpt because multires doesnt handle very well when you add vertices to the base mesh.
