When you say that the holes in the king post are obviously for the 12mm pin, do you mean that they are 12mm holes? If so I wouldn’t worry about adding any sleeves. There needs to be a certain amount of play otherwise you won’t be able to raise and lower the mast. Also the mast needs to be able bend somewhat with varying rig tension, wind strength etc. I would estimate that the equivalent holes on Cockle are about 20mm diameter and I have never given any thought to lining them. (Perhaps that is how they have reached 20mm 😆 ).