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Tijmen-
Reaming should really only be for getting rid of lumps, bumps and distortion from brazing or welding, not changing the internal diameter of the tubing. Reaming that tube back to 31.7mm at any point would be fine. Trying to ream it out further to fit a bigger seat post is asking for trouble though.
On Brommers Bike no. 3 I used a 31.7mm external seat tube, because that fitted the lugs I was using. I bonded a 30.6>27.2mm shim in place to allow me to use a 27.2mm seat post.
Brommers
No. It'll need a 30.6mm seat post or require shimming to 27.2. Reaming it out to fit a 30.9mm seatpost will leave the walls to thin and cause a stress riser at the end of the reamed section.