I would be concerned with the path that the 200 feet of feeder will take from point A to point B, foot traffic, vehicle traffic, ect. Explain the liability of 200 feet of feeder laying on the ground and exposed to foot and vehicle traffic to the event organizers, that may speed up the process of getting a permanent power connection at the stage site. Then again there are event organizers who can't understand why you can't use the provided single, crusty, weathered, questionable if it's even on a single 20amp circuit receptacle!
One thing that would be big cost to adding a 50 amp connection in the area of the stage if by chance the building does not already have a proper power service in it.