I haven't worked with many cantilever brakes, but the ones that I have worked with were designed to rotate directly on the 8mm diameter "braze-on" frame post. Over time, the post can wear out-of round. I have more experience with V-brakes that mount on the same type of post but incorporate an internal bushing on which the caliper arm rotates. There is no relative motion between the bushing and the braze-on post, only between the caliper arm and the bushing.
I am concerned about wear and corrosion of the post and would like to know if there are common canti brakes that use a bushing or bearing to avoid damage to the post. How many of these designs actually use seals to maintain the bushing/lubrication and keep contamination out?