Turns out I need a remote for the Nexa socket. There’s no other way to get the code in RFXmngr.
I discovered that, to get timely power consumption updates from Fibaro wall plugs, you need to use the sensor_power channel, incorrectly labelled as “kWh”, not meter_watts, inaccurately labelled as “instantaneous” (it is very much not instantaneous: it updates every ten minutes or perhaps even less than that).