Yes, fixing the conversion block is probably the right approach.
Apparently posixmodule.c uses PyUnicode_FSConverter, perhaps that would work?
(also make sure that the case where a bytes string is given is fixed too:

>>> open(b"\x00")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
FileNotFoundError: [Errno 2] No such file or directory: ''
)