Nice looking forward to this...I also had a similar idea although mine was a little more complex:

1) Use a command line that takes off the key board lights (You wont need key lights at day time) when Its night you can use a command line to turn on the key lights again. Or better yet have ability to set keypad lights to turn on at a certain time of day (7:00 pm or so).

2) Use a command line to power down the LCD lights to 80% or so...(20% battery increase; I'm not refering to brightness as this is completely different)...also have option to take off lcd lights after a period of time (your idea)

3) Use a command line to under clock the device by 50 or 100mhz. (Dont think the n900 needs the full 600mhz anyway)

Option three isn't nessecary, but the first two options I know would be a real life safer as I did them on a phone (N-Gage) and was able to double its battery life...and trust me, I ain't exagerating. In fact I'm pretty sure if this concept was done on any phone it would surely have a similar effect. This is due to the fact that besides clock rate (600mhz) the lights are the ones taking up the most battery life and they are almost always on, the same can be said for any phone.