Actually, since your milling "both" sides of the post, you're only taking 0.2mm thickness off. The post would probably take it, but I think I'd look for a correct one first. I've sanded a 27.0 down to a 26.8 before. Started with 60 grit and gradually moved to finer grits and steel wool. Looked great but it was a cheap non-fluted post to start with.

How deep are the flutes, and how good do you envision them looking 0.2mm shallower? (May not be able to tell that little amount in reality)