You can connect it to the VIN, but be aware 6V is at the top of maximum voltage supported by the LDO. Just be aware if your battery has a nominal voltage of 6V, you might find out it could have a higher voltage at full charge.
Thanks for the reply. Any idea of how much current the Whisper Node requires when transmitting at full power (LoRa radio). I may run the Whisper Node through a HT7350 voltage regulator, but these are only good for (up to) 200mA.