Only thing like this I can recall is an extra feature on the DVD for "Supersize Me" where the movie maker -Morgan Spurlock- is doing second-unit stuff/research for the movie (after his 30-day McDonalds diet, presumably) and he has a Big Mac and fries sitting on a desk under bell jars, separate jars (not vacuumed the jars are just there for isolation. I forget the period of time but I want to say it's somewhere around 6 weeks and the but the burger decomposes as you would expect (it is, after all still meat, vegetables and other substance that, you know, rot) but the fries remain fine and visually still edible at the end of the period. (Likely due to the high sodium content.)
Stuff like what is mentioned in the OP, to me, reeks of bullshit along the same lines of people saying KFC had to change their name since what they serve isn't "chicken" (it is.) Or that Twinkies last forever (they don't.)
The idea that a burger could survive more than even a couple weeks is preposterous for any number of reasons. It's still meat and vegetables. Organic stuff and it's not greatly protected by whatever salts are in it. It's going to rot. Unless these "20 year old burgers" are inside vacuum sealed containers (which sort of defeats the entire idea) there's no way they'd last unmolested for any meaningful length of time. And dogs eat all sorts of crap. If a dog refuses to eat a McD's burger it's either because the dog is "spoiled" on finer dog foods or some other reason. I doubt there's any "no McDonalds" instinct built into a dog's genetic code. These are, afterall, animals that lick their own crotches and sniff the asses of other dogs.