FWIW:

Trek 2200 alpha aluminum, 50cm, 27.2mm post diameter with approx 7cm exposed, tried it with:
~an ICON aluminum with a ti-railed Selle Italia Flite Gel
~a USE Alien carbon (with same saddle)
~an Easton EC70 carbon (it has 40mm of setback, the Alien does not) with a ti-railed San Marco Era
-->carbon post was a noticeable comfort upgrade here (as was the additional setback, and the Era saddle for me)


Profile Air-Raid carbon, 52cm, 26.8mm post diameter with approx 6cm exposed, tried it with:
~an American Classic aluminum (2-piece, 2-bolt design) with a ti-railed San Marco Era
--> rides smoother than the Trek. I cannot imagine a carbon post being a significant comfort benefit here.

Specialized Allez aluminum, 54cm, 27.2mm post diameter with approx 5cm exposed. A friends bike, he'll let me try it soon. He is using:
~American Classic aluminum (2-piece, 2-bolt design) with a ti-railed San Marco Era
-->The similarity of our configurations makes this an experiment worth conducting. I'll report my findings unless you absove me of it, but I must admit I am curious anyhow.

Idea: what if you packed an aluminum seatpost with sponge-like material (sponge, cork)? couldn't it absorb some of the vibration experienced by the walls of the seatpost?