Mike,
We just had this discussion over on the
Shopsmith forums a few days ago because I'm in the same boat you are.
The bottom line is that there are no commercially available dead centers/cone drives for the SS quill. There are people with metal lathes and CNC lathes who will make/sell them to you.
Perhaps the easiest solution which we discussed was to use a collet chuck, router chuck, or the screw drive (D in your picture) to make your own dead center/cone drive. A few guys said they have had great success with this. If you use hard maple, for example, the brass tube will eventually mar the wood, and you'll have to re-surface the cone drive after a number of pens, but that's not difficult to do.