My 2001 Kilauea, which supposedly shares the same size seat pipe as pre 97 ones, is indeed a 27mm, yet I have fitted a 27.2 Thomson layback. I did this by marking seat height/insertion point on the post, then carefully rubbing down the inserted section with 1500 wet and dry. I didnt have to remove all of the ribbing for it to fit, just the tops of the ridges, so the post still grips inside the frame perfectly fine
I had to do this becasue at the time, although 27mm was available, it was only either straight or silver, not black and layback
G