Is Exposé an app or what?

I want to have my extra mouse buttons behave differently after Exposé is invoked. All I'm really wanting is for something like USBOverdrive to know that I'm in Exposé and send a tab when I click the right button. Are you in a hidden app when you invoke it? If so, and if I can identify it to USBOverdrive, I'm set. If not, is this type of recognition of the state something that is accessible to developers?