I can confirm the LED is not controlled by only keyboard firmware.

I very often use num lock and caps lock responsiveness to check if my PC has hard-locked, and I just recently suffered hard locks while testing S3 on Linux on a new PC. So I am very sure at this point :)

I only really suggested that because any OS can detect the presence of the CAPS LOCK key (see 'CAPS LOCK is on' during Windows login) and it would make a potentially good marker.