Screens that have a separate board for inverter (my xenarc 700idt does), have a control pin in the inverter connector used to control screen backlight.
Normally you get full brigh at 0V and minimum (off) at 3.3V.
Have a look at http://www.mp3car.com/vbulletin/lcd-...o-harness.html
If your lilliput has a separate board for the inverter it will be very easy to get this signal from the four pin connector.
Anyway, i think the photosensor is a very good option for auto backlight control.