GASCo (General Aviation Safety Council) want to see accidents through carb ice become a thing of the past and would like to see changes.
The use of unfiltered hot air is probably due to the difficulty of ducting filtered air to the heater muff. Pressure at the carb intake has to be lower than atmospheric, otherwise air wouldn't flow through the carb. Most hot air muffs leak all around, not a problem for heat because the air being drawn in passes over the exhausts and warms up. If however you tried to draw it via the filter the air would find it much easier to flow through the leaks than through the filter.
One solution would be to fit the filter between the hot air valve and the carb but there's no room in the vast majority of installations.
In practice, while people are worried about unfiltered air, icing requires humidity, which in turn usually means that conditions are not dusty.
I like the idea of hot air automatically being selected when the throttle is closed. It wouldn't be difficult to devise a linkage that would also slightly lean the mixture. You could add a ground/air control enabling you to prevent unfiltered air being selected while on the ground.
The application of modern automotive techniques such as fuel injection and variable ignition timing would greatly help economy. Maybe the rise in fuel prices might help stimulate something in this direction.
Apropos of cars, I well remember having a car with "Winter" and "Summer" settings for the air filter. It clamped on to the top of the carb and could be swivelled so that its intake was nearer the exhaust manifold in winter.