This can be done with display parameters in the shield's item models, which is an ordinary resource pack thing. You don't really need a mod for this.
There's probably already a pack that does this somewhere, but if not it can be done pretty easily with a little know-how. If you can't figure it out, I'm sure someone in Resource Pack Help can give you a hand.
Hey, thanks for this. If anyone else is curious, item/shield.json was easy enough to modify: the left-hand config translation value was lowered from zero to -3.5. Plopped it into my resource pack and it's good to go!