Kind of an interesting question. Initially, I'd say, "That won't work because resistance doesn't tell you anything about the efficiency of the speaker elements in the headphones."
But how about complex impedance? The phone could measure that instead. A perfectly inefficient speaker -- one where you're holding the diaphragm in place with your fingers, maybe, or where you've removed the magnet with a hammer -- will have a reactive component. It'll look like an inductive load if it's an old-school speaker with a voice coil, or a capacitive load if it uses a piezo transducer.
In either case, a perfectly efficient element that radiates all of the incoming energy would look more like a pure resistance, much like a properly-matched antenna at RF. So there might be some room to implement your suggestion, given a bit of R&D effort.
But how about complex impedance? The phone could measure that instead. A perfectly inefficient speaker -- one where you're holding the diaphragm in place with your fingers, maybe, or where you've removed the magnet with a hammer -- will have a reactive component. It'll look like an inductive load if it's an old-school speaker with a voice coil, or a capacitive load if it uses a piezo transducer.
In either case, a perfectly efficient element that radiates all of the incoming energy would look more like a pure resistance, much like a properly-matched antenna at RF. So there might be some room to implement your suggestion, given a bit of R&D effort.