Since iOS doesn't support video with Alpha we will have to create our own special compositor to handle these things properly and because iOS doesn't support video with alpha we will have to support it by allowing a matte/fill key. In the meantime there are several green screen keying apps out there that you could use to add your final video animation as long as you could create it with green instead of alpha transparency. I can't say you will get a good key from them though.
We are racing to try to get these things and more done. Since we are an extremely small team we have to always prioritize what comes next. Keying is very high on our list, but we have a big list