After looking through the ShooterGame DefaultEngine.ini, I found that its config notation was different from the documentation:
[/Script/Engine.PhysicsSettings]
+PhysicalSurfaces=(Type=SurfaceType1, Name=Concrete)
As far as I know everything is working codeside now, but, although the entries I manually added to the ini file show up in the Project Settings dialog, I still cannot edit the settings from the dialog. It is only a mild inconvenience to edit the config manually, but it’s something that definitely should be working regardless.