That's a clever idea... but the effort invovled here is fairly significant (the patch would be easy, in theory, but there's a fair bit of work to setup the toolchain to then compile the file).
That said, this inspired me to look to see if there was a temporary lock file or any other indicators of activity... I could simply poll for a file and then turn the LED on or off accordingly. The latter is easy, the former seems like it would also require patching the source code.
I'm now leaning that way... lol. I was thinking it would be a reasonable indicator... but not worth that much effort.
Yeah... looks like it isn't fully supported. I wonder if it ever actually did work in LEDE.
Good find, though!