So to skip the beating around the bush and step-by-step... what I was getting at, Shade, was that there's nothing to draw intelligence from in ModelSpace that would tell you HOW to scale the linetype so it shows as it does on paper. You'd have to pull the plot scale from somewhere.
That's what the annotation scale is. It's just that Autocad is not designed to use the annotation scale to display lineweights in modelspace.
But the annotation scale is a new feature in 2008, so if you pester Autodesk about it enough, they may add the ability in a future version.