In such cases, I would first confirm that a typically-undersized standard post (in this case 25.8mm) does not fit with a modest bit of pushing and twisting. SR LaPrade and others have made this size in a fluted alloy post, and my Raleigh Technium 753 Team frame uses this size. I now have a very nice, non-fluted, Nitto post in it. Early Marin aluminum frames and some cheaper 1970's lugged steel frames have used this size also. Assuming a 25.8 post really doesn't fit, one could try sanding one down. I have found fluted Stronglight posts in odd smaller sizes like 24.0mm, and perhaps there are others.
David Snyder finding seat tubes relatively difficult to measure in Auburn, CA
From: "Sean Flores" Subject: [CR]25.6 French seatpost, is there a chance?
> So I recently picked up one of the Fraysse¹s old Paris Sport track frames.
> It was built in the 70¹s by Dangre Starnord, but oddly repainted by and
> rebaged in the 90¹s By Cuervas. Paris Sport DT decals, Cuervas Spain HT
> and
> ST decals. Either way, I love the frame.
>
> Today I took it to my LBS to ream and measure the seatube. Diagnosis: 25.6
> argh!
>
> So, I¹m in the market for a Stronglight or Simplex seatpost 24.6 diameter,
> preferably fluted. Worst case scenario is that I have a 25.0 simplex post
> that I can shim, but I would really like to have the correct size and rid
> myself of the Kalloy that currently graces it.