I don’t know why he would use LevelStreaming to avoid the small “bug” of being in 0,0,0.
As long as he doesn’t actually need levels to be streamed it’s not necessary.
The earliest entry point, in Blueprints, is probably the BeginPlay of the LevelBlueprint.
You can try showing a Widget there. If that’s still showing the 0,0,0 Camera location for a milisecond, then you would need to actually use C++ and implement a proper loading screen. Instead of removing that on “PostLoadMap” you can delay it to remove it when ever you want.