I would think that corrosion would mainly be a factor with combustion liquids condensation (Ie low temps) or short duration useage by not coming up to full temp. Erosion would be from high temp corrosive fast moving gases and slower to harm. I have never heard of Alum mufflers before. I would think a Ti muffler would be doable, albeit at high but reasonable cost, even home made. Sounds interesting. And remember that alum loses a lot of strenght at even 400F, a small backfire at operating temp and I would bet an Alum muffler would open right up. Alum with a decent amount of continuous outside cool air flow I guess is needed for durability. Header wrap would likely be no-no, but you could also jet coat it inside and out (if no packing) for additional lifespan.