I have found relevant documentation in a comment preceding get_path_importer definition:

   ... traverse path_hooks until a hook is found 
   that can handle the path item. Return None if no hook could; 
   this tells our caller it should fall back to the builtin 
   import mechanism.

Therefore, Py_None return is legitimate and should be handled the way I 
suggested in my previous message.  The same comment explains that 
get_path_importer "Returns a borrowed reference," so my criticism of 
return Py_None was misplaced.

I will submit a documentation patch adding PyImport_GetImporter 
documentation.