I broke off the little tab visible in this picture between the webcam LED cover and the sticker. It's in just the right spot to put an LED without hitting the Wi-Fi antenna or the webcam.
Then I drilled a 1/8" hole in that spot, at an angle.
And cleaned up the edges of the hole.
I harvested a T-1 white LED from a string of LED Christmas lights.
And a pair of 30 AWG stranded wires from a round IDE cable.
The LED fit snugly into the hole and is secured with hot glue.
I clipped the leads short, soldered wires to them, and tacked the wires along the side of the bezel using little bits of hot glue. The positioning of these wires can be a little tricky because it's important that they not get in the way of the bezel clipping back on.
When assembled, the wire comes out through the left hinge.
I soldered a 68-ohm resistor directly to the +5V pin of a USB port (pin 1 of connector JP7). This way there's a minimal possibility of a short, but if a short does happen, the USB controller can cut off the juice. The other end of the resistor is connected to the wire going to the anode of the LED.
A microcontroller manages the LED. This is an ATTiny13 (in its 0.150" SOIC form it's the smallest AVR that can be easy handled by a hobbyist). +3.3V and ground are taken from JP14 pins 1 and 4 respectively. The ground pin is soldered directly to the pad and the Vcc pin is connected using one of the leads cut off the resistor. These two are sufficient to hold the chip in place. The chip is also connected to pins 1 and 11 of the keyboard (so it can monitor the Ctrl key) and to the cathode of the LED. Code is in the comments below.
A view of the entire motherboard, modifications in place. You can also see that the power LED is covered with a few layers of masking tape.
This picture doesn't do a good job of showing how effective the LED is, but here it is anyway. You hold the Ctrl key down for a few seconds to toggle the LED on or off; this works independently of operating system or BIOS.