The whole screen is game to draw to as long as the bounding box is in the view.
I haven’t explored c3’s renderer but it’s likely a batched renderer similar to C2. As such you’re limited to features exposed to to the renderer. Drawing to a texture is probably just an undocumented feature. C2’s renderer had it as a feature at least. It’s used for drawing effects and layers with force own texture. The paster plugin utilized this, although there is more setup and such.
Now in C2 there was also a function in the renderer to complete all current batches. This was useful to let the renderer finish what it was doing and then you could do anything you desired with all of webgl as long as you preserved the webgl state to what construct had it at. This can kill the benefits of batching so I doubt this way would be officially supported. C3 probably has a similar undocumented function in its renderer.
Examples in C2 are the dragon bones plugin and some more recent 3D experiments I posted capx for.
Anyways, webgl provides a couple clipping ideas. One is scissoring. It limits what is drawn to a rectangle on the screen.
The other is stencil buffers. You can draw anything to the buffer and then you can draw to the screen like normal and the stencil acts like a mask.
Anyways just some ideas.