<-socal melee player, been going to tournaments for 2+ years
I've thought about this question a lot, and i have the most experience with going to tournaments hungry as ***. most tournaments i wake up from a smashfest, eat some skittles or a gas station hot dog at 10am, and then don't eat until after the tournament at like 1am. I might get some hunger rumbles half way thru the tournament, but when i'm focused and playing, i don't feel anything.
this usually works alright for me, but if you have "crazy eating habits" and gotta eat during the tournament, i suggest you bring in your packpack 2 apples (if you're pro, green, cuz green ****) and some bananas. and some beef jerkey.
DONT EAT PIZZA. i swear to god the worst i've ever played in my life was the first half of the san diego triweekly a few months ago. i was really hungry before the tournament so i went to the school's store and bought two big *** slices of pizza. it was tasty. but once teams started i felt TERRIBLE. i was full of crummy greesy cheese and my fingers were slow, i couldn't think. part way thru singles i felt a lot better (and did really well) but i learned my lesson for sure. don't stuff your face full of greesy junk food right before you play. you will feel terrible and play terrible.
so I reccomend if you gotta eat on a tournament day, try to get a decent breakfast with some semi-healthy things (if you wake up at home before the tourny, make some legit eggs and toast.) if you can't get ahold of any small fruits or beef jerky it's ok to eat junk food, but in small portions. if you stuff your face with McDonalds hash browns right before teams you're gonna get *****
good luck