There wouldn't be any need for pressure sensors; this already happens. The charged device adds some power to its own coil and the charging station detects that (it's unidirectional communication). For example my charging station lights up about a second after putting my phone down, and can show the battery status with the light. I would assume that when there's nothing on them the coils transmit in a low-power mode, with the protocol allowing the device to ask for more/less power as required (edit: Wikipedia confirms that the device can actually send some pretty precise instructions to the station, with voltage controllable in 50 mV increments [0]).
So logically it would seem the heat issue happens when there are three coils active at the same time transmitting at high power - the peak heat output is the issue.