Curved & Animated Mask

I have a curved graphic that I need masked, but the mask will slowly unveil the graphic below. I’m tweening the mask on the timeline since the graphic has a pretty complex shape (a bunch of curveTo’s would take forever).

If I apply a shape tween to the mask it doesn’t tween on curves that well unless I place a keyframe at every two frames and adjust the shape. Does anyone know if there is a better solution?

Cheers,
ghjr