I think I saw an nes homebrew that displayed a qr code linking to the developer's website. It was a tdch demo or something. Should be feasible on an atari 2600 using only playfield graphics (maximum 40 pixel width, and many codes are smaller than that).
This is fun and interesting, although compared to a Harmony cartridge it is of course pretty clumsy.
A much more interesting and useful application of QR codes goes virtually undetected in new games for the 8 bit computers. The guys from Poland have put software in a number of recent games like Stunt Car Racer and Bosconian that generate a QR screen on your TV/monitor. Scan that and you get a registered high-score on a website.
No extra hardware needed, the Atari 8 bit computer simply displays a lot of black and white blocks....which is a QR code