Yeah, I think this sort of thing would be difficult to pass on in the same fashion today. With the advent of digital, one of the major reasons people have for buying print in the first place is the collectability/preservation aspect, which cutting off part of the book would obviously undermine. And books cost a lot these days -- I'm pretty sure comparatively more than they did forty years ago -- so it'd be hard to justify buying two copies, one to keep and one to mutilate.
I agree that something with codes or QR inside the book (much the same way they currently deal with digital copies in Marvel books) would be the way any promotion like this went today. It'd also be interesting to see whether this was opened up to digital readers, or whether it (like many things) would be exclusive to people who bought hard copies of their comics.