Why would you ever need to set the /etc/inittab to RL 0 or 6? Wouldn't you want your system to come back up after the reboot?
init 0 actually is a symlink to the init 6 script because it's similar init 6 reboots the system except when invoked as init 0 in which case it halts the system, (Powered Off, no activity, can be safely powered down)
Emergency reboot or shutdown with magic commands
Force an immediate reboot:
echo 1 > /proc/sys/kernel/sysrq echo b > /proc/sysrq-trigger
Shut the machine down entirely: (Very rarely used)
echo 1 > /proc/sys/kernel/sysrq echo o > /proc/sysrq-trigger