I don;t see it as an authority issue, but all in good fun. At many school fundraisers principals will do similar things: pie in the face, shave head, shave off beard when the school reaches their target goal.
For our pie in the face we had a painters tarp on the floor and hanging in the back. We did use cheap pie tins but filled them with spray whipped cream (at leaders request). Scouts then shot from a line and all at once. It worked great. Very little whipped cream went out of bounds. Leaders could wear a poncho if they wanted, goggles too.
At ours the top popcorn seller in each den pied their leader. The top seller of the pack got to pie the leader/cubmaster/or volunteer of their choice. Everyone knew in advance who was to be pied.