Any mods that allow the character to wear their shield on their back while it isn't being used, either through script or just a piece of armor that resembles a shield? I'm aware of a few mods that allow sheathing of weapons, but I've never seen one that does this.

As far as I am aware, no such mod exists. You'd need to have meshes for all shield "back" variants, scripts for it, etc and I'm not even sure if the shield slot would allow you to put the shield on back.

Sandman101 and That Bloke are working on one as we speak.

I'll be back with a link to the thread as soon as I can locate it.

Edit: http://www.gamesas.com/index.php?/topic/1158818-wip-evolved-armamentarium/

KF