Normally dumping even when pulling up does not overstress the aircraft. Do all the maths you like, doesn't make it so. Talk to an Ag pilot, they actually know. Take the standard waterbombing aircraft now, an 802 is stressed to lower G than a Pawnee, the job is to dump water on fires. The Pawnee referred to was deficient with maintenance which caused catastrophic airframe failure. It is a silly thing to do gender reveals in any way let alone an aircraft.