Imagine how offended you might be if you did something that generous for your employees, AND made sure they still got paid for the day, and then found out that they expected overtime if the event took more than the normal working hours. Would you want to schedule something fun like that for them again? Would you have the same attitude towards those employees again?
Bottom line, your employer is paying for the cost of the fun, and still paying your wages. While there might be some technicality that requires them to pay you overtime since attendance at the event is required, you might want to consider whether or not insisting on the overtime is a wise move, politically.