Since no one had any specific part recommendations,
, I went ahead and ordered the choke and 7 different EMI filters that I linked at the beginning of the thread. I probably didn't do a good job spec'ing or choosing them, so it's very nearly a shot in the dark. Be that as it may, now that I'm able to read ambient RSSI on the RFM69 all the way down to -127, I'll try them out as best I can and see if any of them yield noticeable improvement when running from a DC voltage source that's mains powered and with other possible interferers on the same circuit. I also have a rig I can use for testing purposes to insert components between the radio module and the atmega328p and other board components to test the choke and filters for effect if used in that way.