> Ну ё-моё. Рабочий код:
> import os; fd = os.open("/", os.O_RDONLY|os.O_DIRECTORY); os.chroot("/tmp"); os.chdir("/");
> os.fchdir(fd); print os.listdir(".")

NetBSD fchdir(2):
     ...
     fchdir() will fail and the current working directory will be unchanged if
     one or more of the following are true:
     ...
     [EPERM]            The argument fd references a directory which is not at
                        or below the current process's root directory.