Having searched for a way to enforce immutability of custom types and not having found a satisfactory answer I came up with my own shot at a solution in form of a metaclass:

class ImmutableTypeException( Exception ): pass

class Immutable( type ):
   '''
   Enforce some aspects of the immutability contract for new-style classes:
    - attributes must not be created, modified or deleted after object construction
    - immutable types must implement __eq__ and __hash__
   '''

   def __new__( meta, classname, bases, classDict ):
      instance = type.__new__( meta, classname, bases, classDict )

      # Make sure __eq__ and __hash__ have been implemented by the immutable type.
      # In the case of __hash__ also make sure the object default implementation has been overridden. 
      # TODO: the check for eq and hash functions could probably be done more directly and thus more efficiently
      #       (hasattr does not seem to traverse the type hierarchy)
      if not '__eq__' in dir( instance ):
         raise ImmutableTypeException( 'Immutable types must implement __eq__.' )

      if not '__hash__'  in dir( instance ):
         raise ImmutableTypeException( 'Immutable types must implement __hash__.' )

      if _methodFromObjectType( instance.__hash__ ):
         raise ImmutableTypeException( 'Immutable types must override object.__hash__.' )

      instance.__setattr__ = _setattr
      instance.__delattr__ = _delattr

      return instance

   def __call__( self, *args, **kwargs ):

      obj = type.__call__( self, *args, **kwargs )
      obj.__immutable__ = True

      return obj

def _setattr( self, attr, value ):

   if '__immutable__' in self.__dict__ and self.__immutable__:
      raise AttributeError( "'%s' must not be modified because '%s' is immutable" % ( attr, self ) )

   object.__setattr__( self, attr, value )

def _delattr( self, attr ):
   raise AttributeError( "'%s' must not be deleted because '%s' is immutable" % ( attr, self ) )

def _methodFromObjectType( method ):
   '''
   Return True if the given method has been defined by object, False otherwise.
   '''
   try:
      # TODO: Are we exploiting an implementation detail here? Find better solution! 
      return isinstance( method.__objclass__, object )
   except:
      return False

However, while the general approach seems to be working rather well there are still some iffy implementation details (also see TODO comments in code):

1. How do I check if a particular method has been implemented anywhere in the type hierarchy?
2. How do I check which type is the origin of a method declaration (i.e. as part of which type a method has been defined)?