Thanks John for that bit of information about cinelli seat tubes (they apparently have an internal sleeve brazed-in or a piece of truly double-butted tubing was used for the cinelli seat tubes.)
I believe that this type of crack often happens when you raise the seatpost up very high, so that not enough seat post extends down into the seat tube, and this puts too much stress on the seat lug, causing a crack to form. Note that, in the photos posted already, the seatpost is already up fairly high, and Unica seatposts are much shorter than campagnolo seat posts :
http://www.classicrendezvous.com/
i'm estimating that 4" (105mm) of an available 6" of seatpost is showing. So perhaps a generic aluminum post from wallbike (300-350mm, 12-14") could double the length of seatpost below the seat lug in the butted area. It could be stripped of its anodizing and a Brooks or Ideale saddle clamp could be used.
- Don Gillies
San Diego, CA, USA