Houdini and Maya are high end, high dollar products. POVRay is just a rendering engine, not an interactive modelling tool. A better option for rendering is probably BMRT, the Blue Moon Rendering Tools, which is a Renderman compliant renderer. It was actually used for several movies. However, like POVRay, it is just a rendering engine, not an interactive modeller.
Nothing is available on Linux for free and with source provided that can do broadcast output.
Blender is really the best thing going in this department, since you can add scripting to it fairly well using Python. It's interface is production quality. As for cross platform, give it up. I think is cross platform. But cross platform graphics tools are pretty difficult to do since such tools are often very happy close to the hardware, and getting close to the hardware on different OS's is not quite so easy a proposition.