Will the home users still dial-in to a NAS? If not, and the home users and the internet connection will be using the same pix interface then I do not believe that it could work, due to the pix not being able to do the routing back thru the same interface.
If the users will still dial-in, then yes, you can do that. Assuming that the NAS sits on a perimiter inteface off of the pix, you could use a nat (dmz) ii diall-in-user-ip dial-subnet and a global (outside) ii x.x.x.x command to do nat/pat for the dial-up users. This eliminates the proxy connection.