When are config nodes defined in the flow editor?

onpaletteadd is called when the node is added to the palette. That happens before the current flow configuration is loaded. So it is entirely expected that eachConfig wouldn't return anything for code called in onpaletteadd.

Indeed. Is this really a one-time only action you want to run, or something you need to do whenever a new config node of a particular type is added/import or removed?