I see Pinarello Dogma and Gan S frames have a proprietary seatpost/seattube combination, which don't have a circular section. An advantage of that design in my opinion is that it's impossible to have the saddle not aligned when adjusting the height of the seatpost. One disadvantage I can see is that if the seatpost is not properly greased and get stuck, you don't have much freedom of movement to leverage on in order to unstuck it. In any case, I was wondering:
Is there any standard for seatposts that don't have a circular section?
Say one wants to build their own frame with this feature. Is it possible to have this on a steel frame? And if it exists, what's the most common standard so that it's easy to find seat tubes and replacement seatpost?