Is there a way to make VideoWriter use the MKV container?

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I know this is kind of a stretch because VideoWriter is so basic, but I would like to use the MKV (Matroska) video container format. It is nicer for our application because each frame is timestamped, and our framerate is not always constant. The AVI or MPEG files like to assume a constant framerate so the video speeds up and slows down if it's not encoded at exactly that framerate.

Does anyone know of a way to make this happen? Or if there's a better solution to my problem?

Thanks