Record video from jit.chromakey

I would like to record my live video after I subtract the background color using jit.chromakey. It seems that there is nowhere I can send the “write” message. I tried to write by using jit.matrix. But it seems that it only write one frame of the video. I would like to write the video for as long as I want. Is there any other way to record/write the video but keep the transparency (after the background subtraction) at the same time? So I can read the video (with transparency) later?