Hi Graynomad and thanks for the extended help!
No, I use resonators on the slave modules. Primarily for cost / space reasons. That's where the rtc output was so useful. Of all the modules, it's probably the power module that takes the most readings, ie several thousand a second. However, the good news is that the data goes into just a few accumulators and the data being sent to the main module is quite limited (ie Vrms, Irms, app. power, real power) so a relatively small array can store 10 seconds of data easy.
Another option is to drop the array data into flash. Given the small size of the SRAM on the 328p's that might be a viable option given that all four variables above are floats and hence take up 4 bytes of data each, right? I don't remember how much of an impact writing to flash has vs using SRAM but it seems wasteful to dedicate almost 20% of the SRAM to this array alone.
It's the main module that accumulates kWhs and there is a lot more Ram to play with on a 1284p. That in turn makes writing data strings a lot easier. But even on the 1284p I'm considering the use of flash ram to store string data prior to batch writing to SD card because it seems like the right thing to do in order to keep the platform as expandable as possible.
So it probably makes the most sense to use a rs485 line to propagate the timing signal and then poll each slave in a ring-around-a-Rosie manner. If I go the flash route, the timing interval between pings becomes almost arbitrary. The only consideration for transmissions that are lengthy is the impact on the module that is supposed to be doing other things too, such as power readings. So I'll likely choose a ping interval between 10-30 seconds.