Roughly how much current would the RTC crystal be likely to draw?
The RTC that I'm using, the DS3234, draws 1.5uA in time-keeping mode. It's I2C brother, the DS3231 draws 1uA. This excludes the spike in current to about 500uA for 100ms when the crystal is doing temperature compensation to maintain accurate time, which happens once every 64 seconds and that interval can be changed up to 512 seconds. Still gives a low average draw of under 2uA. There are RTCs that maintain time in the nA range even with a crystal, with some sacrifice in accuracy. But generally, you can assume 1uA for RTCs.
What's interesting about those numbers is that the RFM69 (according to its datasheet) draws 1.2uA in idle mode (using its rc as the clock), so that part is largely the same (a little better or a little worse for the current drawn in timekeeping mode depending on whichever DS323x you pick). Also, unless I'm mistaken, the mah drawn by the RFM69 to do a time sync with the gateway will probably (?) be less than what the DS323x draws (500uA for 100ms) to do its temperature compensation. Anyhow, if it bears out, it's a somewhat interesting result.
Do we have any info as to the ppm's of the RFM69's rc oscillator? According to the datasheet, its frequency is 62.5kHz "after calibration." According to section 4.3.5, the user can force an rc oscillator calibration (I guess to account for changes in temperature), but for starters I wonder how accurate it is even just assuming a constant temperature.
It looks as though it is possible to configure ClkOut to be the RC clock by setting bits 2-0 in RegDioMapping2, and so it should be possible to directly measure the RC frequency on, for instance, an osciliscope. Unfortunately, it doesn't look as though it can be mapped to DIO0, but there are other RFM69 pins that it can be mapped to. Hypothetically, I suppose wiring it as an external clock into Timer1 would be a fairly good fit (16 bits being 65536 counts) for doing a time sync calibration with the gateway, especially if comparing against highly accurate 1PPS GPS time.... Besides, the atmega328p would only have to wake up roughly once a second to count the overflows, which is manageable, and so it would seem possible to get very high resolution time measurements out of that setup by counting literally every clock tick output by the RFM69 RC. Anyhow, I'm a bit hazy on some of the details, but it sounds promising!