A Visible Mask

Is there a way to have a visible mask, or to at least simulate that effect?

What I’m trying to create is sort of a “paint-splash” effect where I’ve got a “Splat” symbol that 's placed over a target object. I don’t want the “Splat” to go beyond the bounds of the object, so I tried setting the splat’s mask to be the target object. Except then the target object vanishes…

I could just have two objects - the target and a mask, but the potential target objects is unlimited and may be animated.