It's probably not the most elegant of solutions, but in db.py I simply set:
if auth.is_logged_in():
auth.messages.logged_in='Logged in successfully, welcome ' +
auth.user.first_name + ' ' + auth.user.last_name
auth.messages.logged_out='Logged out successfully, until next time
' + auth.user.first_name + ' ' + auth.user.last_name
Kind regards,
Annet.

