I'm no photography buff, but while I assume I could extract the data from timelapse photos, it would be much less time consuming if I could rig a camera (or 3) to record the value directly.
There are several distinct parts to this
1) A timelapse script that runs for months on end with minimal intervention
2) Supplying power to the cameras, and perhaps turning them on and off at specific times.
3) Analyzing image data to determine an absolute illuminance value.
Doing this with CHDK will require some hacking on your part, most likely both with hardware and scripting.
For #1, you might start with Waterwingz "ultimate" script http://chdk.setepontos.com/index.php?topic=9969.0
For #2, you'll find some useful discussion in the hardware hacking sub-forum, particularly http://chdk.setepontos.com/index.php?topic=10284.0
For #3, I am working on code for CHDK 1.4 which gets you the average value of a specified region of the frame buffer. Combined with exposure settings this can give you an absolute illumance value, subject to some caveats. This value could easily be logged to a file. Development thread: http://chdk.setepontos.com/index.php?topic=11081.msg119265#msg119265
* You can measure R, G, B separately or average them, but the color response of your cameras particular CFA is not taken into account.
* The units are unspecified. In other words, a value of X represents some specific amount of light landing on the sensor, but you would need external calibration to turn that into some particular units. You can turn it into something approximating a Canon Bv value.
* If you want to use an existing script for timelapse, you would need to add code to allow it to use this feature. Or if you used the scripts I've developed for this feature, you'd need to adapt them to your application.
This is in the unstable development branch. The feature should be useable, but the interfaces may change from version to version, and you might run into unrelated bugs.