The difference w.r.t. an ERT block is that you can see the formatted result on the screen (but editing through an external editor is not as comfortable as a collapsable inset).
Remember I have the InsetPreview in my back pocket :)I didn't follow the thread further, so I don't know what the question is/was, but it seems perfect that with the InsetPreview, you can generate a preview of an ERT.
Vincent
