Good presentation of the tradeoffs, Mark. Those questions are what had me initially leaning toward composites for the truss; of all available materials, composites have the fewest issues with water, salt, corrosion, or galvanic action. Not to say 'no issues,' but the fewest issues. The only reasons I've veered back towards metals are detailed in the posts above: provability of a one-off composite truss for the expected loads. (Start there: what ARE the expected loads? What are the outliers?) How will a plastic perform under high shock loads and/or long term compression? Basic GRP is good in tension, but compression isn't really its thing.
Certainly someone can build a polymer/composite truss that will withstand any forces a thirty foot boat can deal out. But the longer I looked at the question, the more I realized I wasn't qualified to say what such a truss would look like, nor how it should be assembled.
I would say full encapsulation of most metals in
anything -- foam, polyester, epoxy resin -- is a recipe for trouble. It's where Albin went wrong. "Mild steel should be okay if we just surround it with enough resin." First rule of boats: Water.Gets.Everywhere.
While our truss is most eroded at the top of the resin, it is severely rusted from front to back, top to foot. Had it been stainless steel installed the same way in the 1970s, it likely would have looked fine above the resin level but severely rusted below it. Which adds the danger of false security: shiny above, decaying out of sight. Stainless steel chainplates are the famous example. It's the 1" inside the decks that will kill you.
Aluminum also corrodes in salt water, especially when mixed with nobler metals. But most of its
non-galvanic corrosion is at the surface, like bronze: formation of fluffy oxides & sulfates, a little pitting, but nothing unmanageable.
So where I am right now on the truss:
-- Metal, because metals are known and knowable. Either stainless or aluminum will be fine. Bronze would be fine. Titanium would be awesome. Magnesium would suck.
-- A well-designed truss in wood, composite, dried elephant dung, or concrete would work perfectly; but I don't know how to design or construct a suitable truss from composite or dung. Concrete+salt water+metal fasteners offers its own set of nightmares. And while I work with wood professionally, I also have some curious superstitions about where it belongs. Our house is entirely steel, for example.
-- Observing the boat axiom Water.Gets.Everywhere (with the corollary "Water wrecks everything eventually"), our plan is a corrosion
resistant material with good air circulation, careful water-ingress prevention measures, enthusiastic drainage scheme for when prevention fails, and easy access for inspection & maintenance.
We're probably overthinking this whole issue, and I believe there are any number of equally correct solutions. But the known consequences of getting it wrong cause a worrier (like myself) to turn over the matter endlessly.