That would be simple enough to implement yes 
But if it’s done through the velocities then it might introduce clipping either to the maximum or minimum value. Which would affect the sound quality 
Better way to do would be to just add a per song volume. This could be similar to per song drumset or per song tempo 
And from settings user could select if the per song volume is used or just the global volume 
@DanielSingularSound do you know if there is such feature request or if we should create one? I think I recently saw something similar
It was this request but that was more about song parts not the global volume level for whole song