Smallfish,
Interestingly enough your water heated manifold wasn't designed to prevent carb ice but to ensure better fuel droplet vapourisation in the inlet manifolds after it had left the carbs!
If ice is going to form it will do so where the depression is; in other words in the carb venturi. To prevent this, most older vehicles had a flap with a "winter" setting that caused the carb to draw air from around a shield around the exhaust manifold (just like aircraft). Some car engines produce noticeably less power when this is selected. I drove a 4wd in NZ that wouldn't pull the skin off a rice pudding with hot air selected in winter. Fortunately I never suffered carb ice with it at cold.