I have my own image built with LEDE and it can be controlled. Not tried in the factory image.
Luci:
system/led
add LED Name: gl-ar300m:system
trigger: none
This switches it off.
You can configure it for whatever you like. I loaded kmod-ledtrig-heartbeat and have it flashing to indicate a successful boot. You could also set it to indicate wan,lan or wireless data or even a USB active.