I'd be interested in an OOK mode too. It should not be very complicated, however if I understood the specs correctly, an additional signal output (DIO1/DCLK) of the RFM69 need to be hooked to a digital input pin on the Atmega in order to be able to decode arbitrary bitstreams, ie. not packetized ones or at least not the ones the RFM69 can be made to receive. OOK packet mode is inferior to the FSK mode, IMHO, and it's general usability lies in the ability to receive transmissions of cheap weather stations, radio remotes, etc. For those it's probably impossible to write an API that supports every existing data format, but a good general OOK bitstream interface for continuous mode would be very useful, I think.