fwiw, the slenderest shaft I've successfully turned with a centre, otherwise unsupported, in a slant bed cnc, was 15mm bar down to 1/2" over 13" long. 316L. I had one end in the chuck with excess material, as suggested by Matt above.
On the advice of our sandvik rep, I did it in a single pass using a VBMT with a .2mm rad. I was extremely skeptical, but it worked a lot better than I expected, barely noticeable chatter and about .003" big in the middle.
That was a long time ago on very nice Gildemeister lathe, with an exceptionally smooth, fast, ceramic bearing spindle and fantastic accuracy. I would not like to attempt the same on any of the lathes I have now. I was spoiled by that lathe, the new lathes that we have bought since have been bit of a harsh reality shock in terms of accuracy and capability.
These days I put such parts directly to the sliding head guys, don't even think about doing it myself.