I use the "offline mode" and this makes a big difference to battery life when just leaving the N800 idle. (But then of course you are completely offline)

I imagine the combination of going offline and locking key input will put the unit into the mode which would reduce battery drain to a min. (Taking it close to the Nokia 770 with cover on etc)

Note: If you lock the keys you will not be able to switch off or snooze the alarm quickly.