By googling openfirmware forth shutdown, ...
Google gives http://mac.linux.be/content/guide-
<blah, blah> which tells me to try shut-down. No dice.
So try googling '"mitch bradley" "shut-down" power'. Bingo! The twenty-first link is entitled "Re: GRUB on OLPC/XO". Therein we find power-off. That's it, power-off.
More deductive souls might turn to http://wiki.laptop.org/go/Forth_Lessons
and in lesson 5 find the command sifting. With an inkling that power might be involved, "ok sifting power" yields about a dozen commands. Near end is power-off. Bingo again!
Regards, ... Peter E.