So, today I needed to scan something. And our Canon MP560 printer was out of ink... The beast never got past a screen saying "Ink tanks empty" on this and that colour, and thus wouldn't let me scan. Even remotely from my VueScan. And it was Sunday so our local friendly printer ink recycling store was closed. I suspected all hope was out, and cursed printer manufacturers conspirational grip on customers' use of their products, but did a quick google nevertheless. Lo and behold!
"If you want to scan during an ink tank / cartridge error condition, press and hold the <Stop> button on the printer for at least five seconds, and then release it. The above procedure will disable the ink level detection. If the issue still persist, the ink cartridge would need to be replaced." (Source)
I put back the empty tanks. Turned on the printer. Held the red button (that I guessed was) called "<Stop>", and hey presto, I could scan.