So… I think I figured this out today.
const UFoobarSettings* Settings = GetDefault<UFoobarSettings>()
… is going to load DefaultFoobar.ini if Foobar is in MyGame/Plugins/ (that is, if you’re developing/programming Foobar) but if Foobar is in Engine/Plugins/, it actually won’t load Engine/Plugins/Foobar/Config/DefaultFoobar.ini. Instead, you have to have your settings in Engine/Plugins/Foobar/Config/BaseFoobar.ini.
I don’t quite know why this is the way it is or if it’s a bug or if it’s documented anywhere, but that seems to be the way it works.