Is there an event triggered when a form is shown?
From the base model code:
def view_init(self, cr, uid, fields_list, context=None):
"""Override this method to do specific things when a view on the object is opened."""
So in any model, you should be able to override view_init, and put any custom code in there. You don't even need to call super(), because the base class's view_init doesn't do anything.