First, let me state that I like the idea of a uniform approach to 
importing, but given the complexity of the task, Brett's approach (which 
I understand as implementing the builtin importer classes in Python) 
would make sense (as long as he can solve the chicken and egg problem).

This said, here is my first question:

Is there a reason why new importer classes are not subclassable while 
zipimporter is?

>>> class x(imp.BuiltinImporter):pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: type 'imp.BuiltinImporter' is not an acceptable base type

>>> class x(imp.DirectoryImporter): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>

>>> class x(zipimport.zipimporter): pass
... 


Users might want to overload find_module while reusing load_module from 
a built-in importer class.

In fact, I see a great deal of code duplication between the new importer 
classes that can be reduced by deriving from a common base class.