Very feasible, it is how your TV remote works... However, current-gen iPhones have IR filters on their normal (i.e. non-selfie) cameras, so this technology can't be applied through a firmware update.

I think it could be made more effective using radio waves.

Not difficult. It could just be a cheap FIR filter over the output of an IR photodiode.

Of course, if Apple actually used it, they would lose a lot of customers (starting with me).