Im not sure about built-in webcams, but I think it is most likely possible.
I've found info on Logitech Webcams, where you can turn off the LED in the registry keys..
For QuickCam versions 22.214.171.1249 thru 126.96.36.1993, LVUVC_LEDControl is located in the following registry key:
Note - If more than one camera is installed, you will have a "folder" for each device (i.e., 0000, 0001, 0002, etc...).
It has a default Data Value of REG_DWORD = 0x00000005 (5).
Based on your comments, I will assume that the following information is true:
0x00000000 (0) = LED Off
0x00000008 (8) = LED On
Please note that only certain cameras support this feature, so the mere addition of this key will not cause the LED Control buttons to appear in the QuickCam® Advanced Settings.
It is taken from: http://forums.logitech.com/t5/Webcams/Can-I-turn-off-red-LED/m-p/277305#M52816
Hence, I do not see why it shouldnt be possible for built-in webcams, since they also require drivers (mine is for example up side down after a fresh install)