>  { path: [string, bytes, int] => path_converter => path_t },
>  *,
>  { dir_fd: [int, None] = None => OS_STAT_DIR_FD_CONVERTER => int },
>  { follow_symlinks: bool = True => "p" => int }

Why not just:

  path: path_t
  *
  dir_fd: dir_fd_t = None => DEFAULT_DIR_FD
  follow_symlinks: bool = True => 1

?

And register types somewhere:

clinic.register('path_t', restype='path_t', converter='path_converter', signature='[string, bytes, int]')
clinic.register('dir_fd_t', restype='int', converter='OS_STAT_DIR_FD_CONVERTER', signature='[int, None]')
clinic.register('bool', restype='int', converter='_PyBool_Converter', signature='bool')
...
clinic.register('string', restype='PyObject *', converter='_PyUnicode_Converter', signature='string')
clinic.register('buffer', restype='PyBuffer', converter='_PyBuffer_Converter', signature='buffer')
...
clinic.register('int', restype='int', converter='_Py_int_Converter', signature='int')
clinic.register('unsigned long', restype='unsigned long', converter='_Py_long_Converter', signature='int')

If you use path_converter, then definitely input types are [string, bytes, int] and an output C type is path_t. You need only name all converters and register them.