I'll tell ya when its cheating...when I get lazy and rack a cider straight into a just emptied keg (also that had cider) without cleaning it or sanitizing it. Used the same yeast, kept at fridge temps, we'll see if my laziness gives me vinegar, I'll deserve it if so!
Seriously, I've done a lot of wild and wacky stuff as a homebrewer that I consider very, very unauthentic, but occasionally interesting. I like authenticity but I like experimenting too. The rules change, of course, when you start selling and marketing a beer, or when you enter into contests judged by guidelines. But if you and the mooches who drink your beer are all you have to please, do whatever you want to do to get your beer tasting good. Look at the Aussies with their BIAB methods, and Sam Calagione and his...well...creative recipe formulations. Remember that centuries ago all the wacky things those Flemish blokes did was probably considered VERY inauthentic, right?
"What, Henrik? You are putting that sack of sour cherries in our ale? What new madness is this?"