Proposed preliminary patch adds three functions in the codecs module:

convert_surrogates(data, errors) -- handle lone surrogates with specified error handler.

>>> codecs.convert_surrogates('a\u20ac\udca4', 'backslashreplace')
'a€\\udca4'

convert_surrogateescape(data, errors) -- handle surrogateescaped bytes with specified error handler

>>> codecs.convert_surrogateescape('a\u20ac\udca4', 'backslashreplace')
'a€\\xa4'

convert_astrals(data, errors) -- handle astral (non-BMP) characters with specified error handler.

>>> codecs.convert_astral('a\u20ac\U000e007f', 'backslashreplace')
'a€\\U000e007f'

Names are discussable.

I think also about adding two functions or error handlers (that can used with convert_surrogates and convert_astrals) for composing astral characters from surrogate pairs and vice versa.