Are there any methods so that I can search for all users who earned the particular hat(s)?
Is Data Explorer helpful for doing those by writing some new queries?
Searchable leaderboard is available; however the search is by user name, not by hat name.
I don't think this is something we could realistically put in the data explorer. It would be a bit of a challenge because the event is very short-lived, and we'd have to do some interesting hackery to get the data into data explorer itself (creating the dump, dealing with the schema, etc).
However, I'll see if we can include some interesting data (most popular, least popular, most common, rarest, etc) when we wrap the event up in January.