I think an update is in place here.
I did in fact succeed with the 375 km challenge at 27,2 kph. And I felt pretty comfortable during the race.
I never bought a new seatpost.
But I did take the chance and bought a new saddle, with only 200 km to test before the race itself. I choose this
as it seemed a good budget solution even though it's not a WW saddle at 225 g. I will never go back to saddles without a cutout, and now I can explore lighter alternatives
I also supplied the bike with Conti GP 4000s 23 mm and micheliin latex tubes, and the right pressures according to my weight.
During the race I had the oppurtunity to watch several large riders with thin carbon seatposts and I witnessed lot of flex at least 5 mm in each direction, so at least with thin seatposts and long clearance there seems to be some suspension to get from a carbon seatpost. And my guess is that you will even get some from a shorter one with a larger diameter.