Is it the case that given a filename, it might be possible to load a module even if open(filename) fails?

I think the logic is clearer in the form where it is not pulled out into a separate helper function.  You can avoid the double check on the extension by doing:

    if filename.endswith(importlib.machinery.BYTECODE_SUFFIXES):
        loader = importlib.machinery.SourcelessFileLoader('__temp__',
                                                          filename)
    elif filename.endswith(importlib.machinery.EXTENSION_SUFFIXES):
        loader = importlib.machinery.ExtensionFileLoader('__temp__',
                                                         filename)
    else:
        loader = None

    if loader:
        xxxxx
    else:
        xxxxx