Before the current patch 1.24, I could use vanilla character proxies to add new slots. For example, I created a "Headphones" slot that went into its own unique character slot, but worked on the headgear proxy.

class CfgNonAIVehicles
{
	class ProxyHeadgear;
	class ProxyHeadset: ProxyHeadgear
	{
		scope = 2;
		inventorySlot = "Headset";
		model = "DZ\characters\proxies\Headgear_DZ.p3d";
	};
};

With the release of 1.24 this functionality is broken, and to add a new slot to the character on his proxy, I have to rewrite the proxy model of the character, which is not good.
If this functionality is returned, it will allow mod developers to use on vanilla proxies, adding for example: watches, kneepads, rings and other visual attributes