There is the latent question of how this works with B&W printers which are still frequently found in offices. I assume that all of these have a similar technique, just sans the colour. I did notice some scattered black dots on a Kyocera printout (FS1000 or so), but with an older laser printer it's hard to tell whether these were actually made by the printing engine, or just spilt toner trickled down by vibrations. These were rather obvious.

However, I - and evidently many others in this thread - can think of many B&W ways to hide data in a printout.

By the way, if someone wants to take a stab at an older printer's firmware — many Kyocera printers from the late 90s and early 2000s used some small PowerPC with the firmware on a mask ROM on a SIMM-like module. Doubtful that there is anything protected there.