We did exactly that four years ago with JanOS (http://janos.io/), it turned (Firefox OS) mobile phone motherboards into IoT devices. You could also control some GPIO, see http://blog.telenor.io/gonzo/hardware/2015/02/10/gpio.html for an example of remapping the volume buttons to control a new LED. Unfortunately we couldn't find a single OEM that wanted to sell the motherboards if they weren't going in a mobile phone, and ODM's didn't want to work with us either if we wouldn't build phones.