In the short term, you could write an intermediate login module that implements authenticateUser, by calling the seam login module with a new callback handler (that you provide), and that handles the UsernameCallback and PaswordCallback, by returning the values set for _username and _password (by the AppservPasswordLoginModule).
we can also look into making pluggable realms more jaas friendly. As you know they use the jaas interfaces in a "special" and yet public way; which we need to honor for backward compatability reasons.
if your focus is authentication in the web tier. you might find it more effective to implement a jsr 196 server auth module according to the servlet profile of jsr 196. Such modules have complete control (and responsibility for) the message authentication dialog, and jsr 196 defines a login bridge profile, that describes how a server auth module can interact with a jaas login module. The 196 standard provides callbacks for returning groups and the caller principal to the container.
Ron
http://blogs.sun.com/enterprisetechtips/entry/adding_authentication_mechanisms_to_the
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