An update from Neil, who writes:

Removed unicodefilenames function as semantics unclear on other
    platforms.  Commented on future compatibilty with 4 byte Unicode.

pep-0277.txt

@@ -66,12 +66,6 @@ Specification
     this proposal, it will return a list of Unicode strings when its
     path argument is Unicode.
 
-    To allow client code to determine that these features are
-    implemented, the unicodefilenames function is provided.  This
-    function returns true when the underlying system supports file
-    names containing most Unicode characters and any valid file name
-    may be passed to open as a Unicode string.
-
 
 Restrictions
 
@@ -89,7 +83,10 @@ Restrictions
     type and Py_UNICODE_SIZE to be 4.  As the Windows API does not
     accept 4 byte characters, the features described in this proposal
     will not work in this mode so the implementation falls back to the
-    current 'mbcs' encoding technique.
+    current 'mbcs' encoding technique. This retriction could be lifted
+    in the future by performing extra conversions using
+    PyUnicode_AsWideChar but for now that would add too much
+    complexity for a very rarely used feature.
 
 
 Reference Implementation