I haven't done the measuring, but if a straight wall 4130 is .9 mm, and the thin middle on standard butted is .6, and if those are the same dimensions on a single butted seat post, then you could safely remove .3 wall thickness from a .9, but not from the single butted tube if that reduced it to .3mm.
The other part of this is can you ream out that much material with a reamer. That is also a little difficult. I have used chucking reamers to reduce tubing by about 7 thou, or about .2 mm. It isn't difficult to make happen, though technically reamers are supposed to be used to remove just a few thou. That can't be true of all reamers since chambering reamers are quite agressive. Still that is what I have heard said from a number of sources. Seat tube reamers are not designed to open up tubing from one size to the next. They ae designed to restore smooth surfaces to your SP after brazing. That is why you can get seat posts in so many sizes. I use a 26.8 for some applications, you can get a 26.6 from a bike shop.