Is Dapper Day in all the parks, or just MK? I will be onsite for Dapper Day this year, but I believe it was scheduled as my Animal Kingdom day.
They have events at Disneyland, Walt Disney World and at DL Paris. If you mean all the parks within WDW, I'm sure that you can dress up and go to Animal Kingdom in your dapper outfit. It's not a truly formalized event. People just show up and go wherever they like. For instance, out here, you'll find dapper people at both Disneyland and Disney's California Adventure, not to mention Downtown Disney. The only official things are the Expo (lots of vendors) and often a dance.
Let me know if you see dapper people at Animal Kingdom.