Can you transform media plates in Composure?

Composure works very well for chroma-keying a green screen and putting that media plate over a CG background (and putting in foreground). But I would like to transform the media plate (move the actor up and down as if riding a horse). And I can’t find a way to do it. I thought I might be able to get the result onto a plane and move that up and down, but there is a huge loss in quality (it looks pixilated). Can anyone help me?

is the composure stack in a shader? is it possible to modify it with adding values to the texture coordinates?