The way I envision it, we wouldn't need the character inventory. All the inventory could fit on the paperdoll screen - since it will all go into your equipment slots. If you're naked, all the inventory you can have is the stuff you carry in your hands (or a rifle that can be slung over your shoulder). Armor and clothing with pockets can give you additional slots, like pants with big pockets having room for rifle magazines. On belts you can put guns, swords and knives - weapons that come with scabbards or holsters. On special belts you can also wear ammo (belts with these holes for placing bullets - dunno how the word for these holes is).
Think Diablo 2's belts: the larger the belt you wear, the more potions you can put in it. I think JA2 1.13 also has this, you can equip holsters and similar things so you can put additional guns and ammo in your inventory.
This way, a character inventory is not even needed: you just have your paperdoll with equipment slots: boots, pants, shirt/armor, belt. Wear pants with big pockets and a belt with holes for ammo, and you get additional slots: two pocket slots from the pants, an ammo slot and a number of weapon slots from the belt.
The party inventory would be an abstraction for all the loot your party is hauling along. It should be based on party strength, for example one party member has 3 STR, the other 6 and another 4, so if each point of STR allows you to carry 10lbs, you'd be able to carry 130lbs that way. I've just made up these numbers to serve an example, but that's basically the point. Also, party inventory would not be accessible during combat so you have to make do with what your characters are equipped with, which leads to a realistic combat-inventory and meaningful resource management (do I put a second gun in my pocket or do I put an ammo clip there?).