For some reason I just noticed this posting.

One partial solution to exfiltrating information by LEDs would be to design the LEDs with capacitors across them, greatly reducing the rate of change of intensity.   This could also be retrofitted after assembly.

In 1977, after building a "Dyna-Micro" Single-board computer (highlighted in Radio-Electronics magazine) I noticed that a nearby  AM radio could easily pick up RF from it.  If anything, this is potentially an even more useable kind of surveillance, because it doesn't necessarily require intrusion into a computer room or building.

                    Jim Bell