Think his problem is that the majority of posts he likes are larger, so shimming won't do the trick. I agree with you though, I wouldn't lathe a post. Even if I was skinny.Don´t shave material from a seatpost mast. That´s a Darwin Award move. Add shims but don´t shave material unless it´s a bike for show (hard to believe but there are those around)
XT posts came in 26.4 but i never watched any for sale on ebay or anywhere else.
But yeah a Kalloy post will last him forever and all it'll require is 4mm of shimming. Which isn't a problem for a lightweight rider. As a fatty I oppose all seatpost shims.