I haven't worked out how yet, but importlib.machinery is managing to bypass the replacement of importlib._bootstrap with _frozen_importlib:

Python 3.3.0b1 (default:8bf691d0b004+, Jul 15 2012, 23:20:06) 
[GCC 4.7.0 20120507 (Red Hat 4.7.0-5)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import importlib._bootstrap
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>>> import importlib.machinery
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>>> importlib.machinery.FileFinder
<class 'importlib._bootstrap.FileFinder'>
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>>> importlib._bootstrap.FileFinder
<class '_frozen_importlib.FileFinder'>
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>>> importlib.machinery.FileFinder.path_hook()("")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ncoghlan/devel/py3k/Lib/importlib/_bootstrap.py", line 1350, in path_hook_for_FileFinder
    if not _path_isdir(path):
  File "/home/ncoghlan/devel/py3k/Lib/importlib/_bootstrap.py", line 117, in _path_isdir
    path = _os.getcwd()
NameError: global name '_os' is not defined
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>>> importlib._bootstrap.FileFinder.path_hook()("")
FileFinder('.')