All things considered, and I know it was mentioned that permanently altering the mold wasn't desired, but a straight-edge and an X-acto blade would/should take care of venting without the need to run a machine. Now that would rely on the hardness of the mold steel, but I would venture a guess that it isn't tempered tool steel. Vent lines aren't anything more substantial than uniform "scratches" that run from the cavity to the edge.
"Why is is called Common Sense when it seems so few actually possess it?"
Guns only have two enemies: Rust and Politicians.