Shape key moving mask

I have a model where I sculpted some veins on the arms as a shape key.
For an animation I want to make it look like the veins start from the upper arms and move forward to the hands not all over the arm at the same time.
Is it possible to dial in the shape key for the veins but mask it so that I can move a mask in the animation?

Or is it in another way possible? I’ve tried the cast modifier but sadly I can only cast a sphere, a cylinder or a cuboid, no custom shape.