// this compiles, therefore the type is clearly accessible
Engine.Logic.LevelDataReader r = new Engine.Logic.LevelDataReader();

// this also works:
var i = Activator.CreateInstance("CEX_Runtime", "Engine.Logic.LevelDataReader");

// this fails, with the exception "Error loading "BigLevel". Cannot find ContentTypeReader Engine.Logic.LevelDataReader, CEX_Runtime, Version=1.0.0.0, Culture=neutral."
            LevelData levelData = this.game.Content.Load<LevelData> ( "BigLevel" );

// here is the runtime reader string:
public override string GetRuntimeReader(TargetPlatform targetPlatform)
{
    return "Engine.Logic.LevelDataReader, CEX_Runtime," + " Version=1.0.0.0, Culture=neutral";
}