How to repair / create a CF seat post clamp interface? I have a carbon Trek Madone 4.7 road frame with a nonexistent clamp interface. At the top of the seat tube, there should be a smaller-radius section of notched aluminum on which to place a clamp. That part of my frame is missing. 

The top of the seat tube is much thicker than any clamp I've seen, not notched, and irregularly shaped. The interior surface has an irregular radius for at least 2cm; I assume that when the bonded aluminum clamp interface failed, it took a layer of the interior with it. 

The rest of the frame is in fine shape. Is there a way I can create a new seat clamp interface? 

I was considering leveling out the inner surface with Shapelock, creating a notch, and sanding down a short external section at the top of the seat tube to be sufficiently regular to fit a clamp.
It's a bad idea to sand down or modify the carbon fiber of the seat tube. You don't know what you will do to its integrity or strength.

I would do some research to find out if a replacement aluminum seat 'cap' that accommodates the clamp can be bought, and the proper technique and epoxy type for bonding it in.

If you choose not to fix the frame, don't junk it. Stick it on Ebay with the appropriate disclaimer about the seat clamp. Maybe someone with the required know0how will but it up and fix it.