I agree, it would be awesome. The runtimes are really the limiting factor. The cheapest way to do clipping in OpenGL is probably a stencil buffer. The downsides are that it would have aliased edges and would cause two batch flushes. None of this matters for exporting video or images of course.
It's a good feature and I'd like for it to make it in, but there are lots of other things to do first.