I am doing something much more simple than that, and implemented it as separate items.
Example:
I have created the following armor items: Chain Hauberk, Leather Armor, Leather armor with Chain Hauberk.
HOWEVER It seems to me that a player that has leather armor, and then finds a chain hauberk, should be able to make Leather Armor with Chain Hauberk by first putting on the leather, and then with a crafting action wear the chain hauberk over the leather.
From a crafting standpoint:
leather armor + chain hauberk = Leather armor with Chain Hauberk
AND this much of the crafting is very easy to do. You create a copy of the base item, modify it as appropriate, then put it on the PC as all standard armor crafting works in NWN.
BUT I have not implemented it yet because I am unsure (1) how best to combine magical/extraordinary item properties for combined armor, (2) how best to track the item properties so that when the base and vest are separated that their appropriate item properties are restored to them. and there is also the issue of item name and description to consider.
With regards to how other things like boots, bracers, gloves, belts, helmets etc... are combined with armor, that is no big deal since the engine gives you different inventory slots for these items. I have considered a system that changes clothing appearance when these items are equipped and unequipped. But that is only aesthetic so its a bit down my list of things to take on right now. Plus there is the issue of performance to consider. Every time a PC modifies armor appearance it creates client side lag for all nearby PCs