There actually is one person who has made a 1/6th shirt with functional buttons.
Oh, I think DiD (or similar) makes all kinds of functional buttoned shirts for their German soldiers. But the scale of the buttons is still oversized is what I meant. The ones on Kim's 1/4th Indy look more closely scaled to a 1/1 shirt in size -- part of the give-away that it couldn't be 1/6th.
Working 1/6th scale buttons are a real pain in the ass to work with though, accurate or not.