Some aeroplanes do not have alternate air.
e.g. UK and USA at least, Rans S6 series.
Interesting, are they certified? Alternate air is a certification requirement. And a fine idea. I did hit very narrow band of freezing rain in the 150 once, and both the windshield and air filter iced over instantly, followed by the engine stopping completely. I pulled carb heat for the alternate air, and got the power right back, as I turned 180, out of what I had thought was just rain!