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.