Unfortunately there is still no masking yet.
To use a bounding box for masking you would grab the bounding box vertices at runtime and use them to apply masking using your game toolkit before rendering the skeleton. Eg, for OpenGL you can use the stencil buffer. In a nutshell, you enable writing to the stencil buffer, render white pixels everywhere you want drawing to happen (eg you'd use the bounding box vertices to render a white polygon), then you'd enable the stencil buffer for masking, then render the skeleton and it will only render where the bounding box is at. Another way with OpenGL is using glScissors, but this only gives you axis aligned clipping. That may be enough and you could still use bounding box vertices to determine where to clip.
