An over-the-shoulder scabbard must be able to move. When you draw a really big sword from behind your shoulder, your arm first moves up, but then it moves in a downward slash (drawing from the shoulder has the advantage of an immediate attack).
So the scabbard should be attached to a shoulder strap at the top of the scabbard. Balance is acquired by attacihing it more or less from the side.

My V4 Warrior set contains a drawing pose, but I do know that the scabbard and shoulder strap aren't modeled to reality. I intend to correct that for my upcoming (think weeks to months) commercial packages.

I haven't found any good pictures either. The M2/M3 Barbarian sets from DAZ have an over-the-shoulder scabbard, but they're not modeled to reality (whatever that may mean in the Poserverse grin).

I'll stick to what I know of human anatomy and physical possibilities/limits....

By the way, that outfit is pretty cool!

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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