Even custom round-tubed carbon is an option:
This is Andy Lakatosh's Parlee. I think he won Silver at Elite Track Nationals in sprints that year (among other things). I also
think he raced Six Days on it. Not sure. He did race Six Days around that same time frame. Having round tubes didn't slow him down.
Parlee's aren't cheap, though.
EDIT:
They are based in Massachusetts.
http://parleecycles.com/
They don't show Track frames in their current model lineup but they mention Track as an option on their Custom page.
Full custom geometry is available on our Z-Zero, Z-Zero DISC, Z-Zero XD (CX, Gravel or Commuter spec), Z1, Z2, Z3, Track, platforms.