overlay_import_loader.py

import sys
import os
import imp

class OverlayImporter(object):
    def __init__(self, path):
        self.path = path

    def find_module(self, name, _path=None):
        names = name.split('.')
        try:
            file, filename, description = imp.find_module(
                names[-1], 
                [os.path.join(self.path, *names[:-1])]
            )
            suffix, mode, type_ = description
            if type_ == imp.PKG_DIRECTORY:
                return
            return OverlayLoader(file, filename, description)
        except ImportError:
            return

class OverlayLoader(object):
    def __init__(self, file, filename, description):
        self.file = file
        self.filename = filename
        self.description = description

    def load_module(self, name):
        mod = imp.load_module(name, self.file, self.filename, self.description)
        if self.file:
            self.file.close()
        return mod
        
sys.path.append(os.path.abspath('normal'))
sys.meta_path.append(OverlayImporter(os.path.abspath('overlay')))