I think more importantly to where you'd put an ambient light sensor, is how you'd hook it up to the logic board. ALS sensors connect to their machines in small plastic sockets and are hardwired into the design of the board in order to provide information and power control to the illuminated setup. In order to make it work, you're going to have to interface with the main logic board in such a way as to allow the system to control the sensor, understand the I/O data, and then shunt the correct response to the backlighting setup.
Honestly, I think its possible to backlight the keyboard by wiring LED's to the right places, but I'm dubious about using ALS sensors. However, if you can make it work, go for it
One last thought. No, there really isnt room in the 12" case for anything else. Its pretty much packed to the gills.