(Not sure if this is in the right section, so just let me know if it should be elsewhere)

Hi! I held a small Morostide contest in the event forums and wasn't able to give out my grand prize to the rightful winner before the forums were closed last night/this morning. I can't remember your username, but If you're the person who won - and you know who you are cause you were super active and you won by a landslide! - please contact me so I can give you your prize. I'd hate for all that time you put into trick or treating to be for nothing. That's not fair. I really hope you see this.

Could their username possibly be in your ping history?

Unfortunately, no. It seems like all of the event-related forum notifications, pings, etc stop showing up in those lists after the event ends. =/