Just wondering if a producer has ever done a scene with a girl on a conveyor getting messed? I know wetandmessy had a conveyor dropping pies on girls.

Model on the conveyor belt? I don't think so. But if you mean stuff dropped from a conveyor onto a model, Wet and Messy had a few videos like that.

A conveyor belt for a human is called a travelator.

I know of one scene here that has one
https://messyworld.umd.net/download_info/the-travelator-with-janey-initiation