[russdill]

Here it might fail. If you were sufficiently motivated and controlled the software stacks on the rpi's you may be able to get data to flow in the other direction. LEDs have their voltage modulated by light. And it's possible that is the voltage on the transistor if properly modulated it may able to emit light. It's a lot of ifs and requires the adc of the rpi to be sensitive enough (and one of the pinmux options). But it's why certifying is important.

Oh, and if you controlled the software stack on the two rpi's there's a good chance there's a side channel somewhere

[wakawaka28]

I reckon it is also possible to set up a second channel covertly at a higher frequency. It may also have surprising flaws. People have read network packets from router indicator LEDs lol.