One other option I was about to play with earlier, before a complete system hang (and work) was to render to double the target size, then use Photoshop to reduce the size down, hopefully anti-aliasing the outline as it went. However, the seperate outline idea might be more useful - you could change the line colours, blur it, sharpen it, whatever you want!

Personally, I hope that in the first service patch they change the toon outliner in Firefly to take account of the preview settings for width and anti-aliasing - the instruction book almost implies that.

Another possibility that occurs to me is to render the toon as normal without the outline at your target resolution, then turn everything invisible and render that at double size, no shadows etc. Scale down the outline to get the anti-aliasing, then merge it with the toon