Why wouldn't you enter it as a milk stout? That's what type of beer it is, yes? I've made a "reese's" oatmeal milk stout and entered it as a oatmeal stout and got a 37 on it. Unless the cereal makes it not taste like a stout anymore, I don't think there's a better category.
FWIW, competitions are generally geared towards traditional beers and beer styles. I love experimenting to make a new taste combination in beer (just made a delicious blueberry stout), but I wouldn't consider submitting most of them due to their uniqueness - there's nothing to base it on for judging.
I entered a Peanut Butter Brown Ale into the same competition and received a 17, because I entered it as a Northern English Brown (since that was the base beer profile, prior to adding the peanut butter), and they said it didn't taste right, was kind of funky, etc. But that was actually the beer we sold out of first (this was at a festival where collectively we served over 25,000 people). Word got around about the PB Brown and people lined up for it. So just a word to the wise to not get discouraged with low scores on "weird" beers. The scores denote how accurate to style it was. There's no style for Peanut Butter Brown, or Reese's Stout.