As far as I know, power saving in the Rx mode isn't easy/currently supported. The RFM69 datasheets talk about a Listen Mode for the Rx where the Rx goes into idle mode (very low power ~ few uA) and periodically wakes up into Rx Mode (16mA). If it receives a valid signal and various configurable conditions are met, it comes out of Listen Mode and you can crack on to receive the message.
There is a listen mode indeed. I have not implemented that mode since it will not really buy you much. I have repeated this many times: to receive ANYTHING you have to be AWAKE (RX mode). That means the whole radio RX end is operating and using power (about 15ma or so). There's no magic bullet to avoiding that. In no radio receiver on this earth.
The listen mode allows you to switch between sleep and RX quickly. That also means you are in SLEEP Nms, RX Mms, SLEEP Nms, RX Mms and so on, you choose your duty cycle. But ... you cannot receive when you are in SLEEP. There, I just repeated that again
So to save space in the library and such, I left that part out. You can always add it yourself OR better... just implement that mode with the existing functionality in a custom sketch. Like ... sleep for Nms, then wakeup and activate the radio RX ... listen for Mms, then repeat. Simple eh? It's the same thing.