Windows has native DPI scaling, which work pretty well. Unless a program uses bitmaps for text, they should scale pretty well. All Parallels has to do is expose the native resolution to Windows, and then you should be able to set scaling there.
You will have a problem with UI elements, etc that are bitmap and which will get stretched and look blurry.
Also, in Windows 8, all Metro style apps should be either using vector based graphics for their UI or least provide a few versions of bitmaps (I think Microsoft guidelines say you should provide 3 versions) so they should scale pretty well.