Dynamic-wind gives us most of the ability to implement parameters ourselves. We just mutate a box upon every entry and exit of the expression. Unfortunately, it might take some crazy trampolining to get the last expression of (parameterize ...) in tail position. I'm not even sure if tail position is possible....
I think the last missing piece is thread-friendliness. In the face of threads, we'd need the box to be thread-local like Racket's parameters. But my point here is just that the pre-thunk is useful for something. ^_^
