I have been experimenting today with an RFM95W hooked up to a Teensy and managed to implement full bidirectional communication to/from The Things Network.

The Teensy by default sends the temperature (from a ds18b20) with every transmit. If you press the button, it sends the button press event instead. Furthermore, on every transmit it listens for a message (which can be scheduled downlong through the TTN dashboard), and blinks the led if a message is received.

The Arduino code running on the Teensy can be found here and the server application code running on the Raspberry Pi here.

Still to be done is to extend the server application code with the button (to circumvent the TTN dashboard alltogether) and to come up with an actual application that is smarter than a button and a LED. I am thinking to link both up and downlink to an IFTTT maker channel.