It's pretty simple. You just need to @Name your subclass the same as Seam's built-in identity component ("org.jboss.seam.security.identity").
(It needs to have a higher precedence, but the default precdence -- APPLICATION -- is higher than the built-in one, so you don't need to specify one)
could you post your example here?
I'm trying to do the same in my application but I don't know how to handle cookies and these stuff.