A cursory search led me to this project[1] that blinks the power led according to the disk activity, which is not far from your idea (replacing the disk activity by a composite of system load for instance?)
[0]https://www.kernel.org/doc/Documentation/leds/leds-class.txt [1]https://github.com/fabio-d/block-led-trigger
Eons ago I had a small program on Linux that blinked the otherwise-almost-useless scroll/numlock leds based on network i/o activity. It was fairly cool.
I just looked at several severs I have access to at several operators, different physical hardware and distros, none of them has a LED defined for anything other than scroll/numlock
As I said elsewhere, none of the (recent) servers I have access to, most of them Intel based, several distros, etc have anything else than LEDs defined for numlock/scrolllock under /sys/class/leds/, where I'd expect to find the CPU activity LED? What am I missing here?
So if numlock and scroll lock are the LEDs available on your hardware, you can repurpose one of those as the CPU activity LED.