Let me throw something at you. Its just something that comes immediately to my mind, might be completely stupid...
If you identify the 3D vertices of your cube, you could use camera.screen() to generate a set of screen vertices that would represent the screen coordinates of the cube in the view. Just in case, the view is the 2D layer into which everything is rendered. With that data you could construct a mask 2D clip. With this, you could use an ¨infinite¨ line and mask it with your clip. You would have to use ownCanvas on the objects and "model.ownSession.getContainer(GameArea.view) as Sprite" to identify the resulting sprite of the owncanvas of the line in order to mask it... It *might work, Li
