Still, may be Yuriy's privious input on a similar problem will be able to help you? The question was "What is the best way to customise the error message when authentication fails?"
Here is the answer:
In your custom script you can do next:
1) Add import
from org.jboss.seam.faces import FacesMessages
from org.jboss.seam.international import StatusMessage
2) Get FacesMessages instance and use it to add more messages:
facesMessages = FacesMessages.instance()
facesMessages.add(StatusMessage.Severity.ERROR, "mesage", params)
3) Also it's possible to remove default error message:
facesMessages = FacesMessages.instance()
facesMessages.getCurrentMessages().clear()
Though, back then there was a problem reported for point 3)
> The default error 'Please use correct username and password' seems to be added to the messages list after the authenticate() python interception method is called. It seems that this default message is added to the FaceMessages list when the result of authenticate() is False