Similar to the decal positioning except its with the shoulder weapons, I think this would allow for some greater detailing and MASS designs as they not are just weapons but a piece that could become part of arms, legs and even backs.
Being able to place the shoulder weapons like accessories is quite an appealing option.
Was actually going to make a post about this myself but you beat me to it haha. yeah I think something similar to the decal sysetem could work. You would say have different positions say 1 to 5. 1 would be default on the shoulders, 2 could be on top of the shoulders, 3 could be on the forearms or somewhere on the arms, 4 could be attatched to the backpack and 5 could be attatched to the legs. Having the option of the weapons being inside the armour parts would also be pretty cool as well.
I would love this too, I would also love to see armor parts with guns and shoulder weapons built-in like in the first 2 pics.