good idea, a tube should spread the tension much better than washers.
You are right, any not perfect strong connection at that part of the boat bears risks to crew and material. Defining the right rivet isn' t trivial at all, not to talk about the
way to fix them. If we talk about 6mm non stainless steel rivets, most hand pliers won't do.
What annoys me @ nacra boats sometimes, is that same parts on smaller / as well as on larger boats are used, although wear is more stronger on an 20ft than on a 18ft.
Same counts for rudder heads/ tillers, jib furlers, etc.

kind regards