The route via gencodec or more generally via charmap_encode and charmap_decode seems to be one that is not possible without some low level CPython code adjustments. The reason for this is that charmap_encode and charmap_decode only seem to support mappings where a single encoded byte corresponds to multiple unicode points.

However iso6937 is a mixed length encoding, meaning in this specific case that unicode characters sometimes need to be encoded as a single byte and sometimes with two bytes.

For example chr(0x00c0) should be encoded as b'\xc1' + b'A'.