Based on the latest round of bytes handling discussions on python-dev, I came up with this updated proposal:

    # Constant in the string module (akin to string.ascii_letters et al)
    escaped_surrogates = bytes(range(128, 256)).decode('ascii', errors='surrogateescape')

    # Helper to ensure a string contains no escaped surrogates
    # This allows it to be safely encoded without surrogateescape
    _match_surrogates = re.compile('[{}]'.format(escaped_surrogates))
    def clean(s, repl='\ufffd'):
        return _match_surrogates.sub(repl, s)

    # Helper to redecode a string that was decoded incorrectly
    # For example, WSGI strings are passed from the server to the
    # framework as latin-1 by default and may need to be redecoded
    def redecode(s, encoding, errors='strict', old_encoding='latin-1', old_errors='strict'):
        return s.encode(old_encoding, old_errors).decode(encoding, errors)

In addition to the concrete use cases David describes, I think these will also serve a useful documentation purpose, in highlighting the two main mechanisms for "smuggling" raw binary data through text APIs (i.e. surrogate escapes and latin-1 decoding).