> a non-ascii group name will raise an error in bytes, even if encoded

Looks like this is a language limitation:

    >>> b'é'
      File "<stdin>", line 1
    SyntaxError: bytes can only contain ASCII literal characters.

No problem if you use escaped character:

    >>> re.match(b'(?P<\xe9>)', b'').groupdict()
    {'é': b''}

There may be some inconveniences in your program, but IMO there is nothing wrong, maybe this issue can be closed.