Remember the N95? It also had a transflective lcd screen just like the N900, but the N95 screen always showed the time. It was really dim but it was cool for most of the situations.

I also saw the 701, which also has an LCD screen and has an Idle screen with the time and notifications just like the N9, with the backlight turned down to the lowest level.

So I think maybe there is not too much impact on battery life if the N900 showed the lock screen always, (with a white background and black text)

I managed to turn the lock screen on when the device is locked with a script that simulates a power button press, and then dim the screen to 2 or even 0, but the screen turns off again 3 seconds later. I made a loop that triggers the power button again, and I managed to make it last for about 15 seconds, but then the screen turns forcibly off, and back on the next time the loop kicks in. The downside is that the backlight is dimmed 0.7 seconds after "power button pressed" so the light flashes once each 15 seconds which is unacceptable. You can try it by physically tapping the power button continuously. The lock screen eventually turns off.

So I am in search of a dbus command or other script that just forces the screen to turn on, and stay that way.

EDIT: various solutions are in development, check the thread. Most stable seems to be woodys' QBW in page 2. (Link)

EDIT: I am happy to announce timenow, a utility that shows the time on the locked screen when the proximity sensor is uncovered. This should be power friendly enough, but please test and report.

EDIT: fixed some bugs and imported to extras-devel