I'm not sure how this would result in more customization though, they could offer the same kind of customization within warband if the initial development of mount and blade / warband would simply have a shoulder slot for example. Currently the combat system allows it to register specific parts of the body that were previously not done in warband, most likely because it would require a fully remade of the hit register system in the engine.The warbands equipment system gives you items in 9 equipment slots. Perk is an arbitrary thing that could be anything, if you could have a dynamic number of perks slots, you could build the same looking/working system as the Warband one, or even more advanced given you add more slots.
-Wanna have more customizability for armor? Apart from helmet/body armor/lower body armor/boots perks slots, add "shoulder protection" perk slot.
-Wanna have a CW mod with some form of control over the character development? Sure, just add attribute perk slots like "strength perk slots", "agility perk slot", where you could buy additional strength/agility given you got enough experience.
-Wanna play some Lord of the rings mods as a small or big troll? Sure, just add "troll age slot", where you select the age of the troll which alters the size of your unit.
TW could be going in the right direction with that system, they just showed us a very primitive version of it. That is at least what I believe in.
Aside from that, the perk system doesn't actually offer more customization for individual slots in your character. In warband it allowed the option to pick from a dozen of helmets or chest pieces, this is what true full customization is.
The implementation of perks and how they behave could have been easily transformed in a system like warband with full customization, and perks slot such a strength or size as you've described.
As an end note, i simply doubt the system was created for more customization. As the singleplayer system for armor is an almost 1:1 copy of warband singleplayer system, with improved things such a horse harness and shoulders for example. If it truly offered customization, wouldn't it have made sense in a singleplayer sandbox game?

