I was DMing for a group of friends and one of them was wondering if he were to wear normal leather armor (11+dex), but wore scalemail (14+dex[max 2]) shin-guards, would a low sweeping attack at his shins have a different AC than normal. I ruled that it's whatever he was wearing more of, which was the leather armor. My real question is: Does it even matter that he was wearing shin-guards? Or is it not possible to "Mix-n-Match" armor?
I think anaximander's comment on another answer is worth expanding into a new answer.
In short, there is no RAW for deciding the AC of custom armour combinations in D&D. Hit location and choice of target does turn up very rarely, most commonly for specific monster, trap or magical effects.
Rather than attempt to use hit locations more generally, which breaks away from a lot of core D&D assumptions, you could attempt to homebrew new types of armour based on your player's descriptions.
I can suggest a couple of rules of thumb:
Any custom armour type should be strictly no better than existing listed items, taking armour category, AC, max Dex bonus, stealth disadvantage and cost all into account. Another way of saying this is that a new homebrew variant should not be obviously superior and first choice for any type of character.
Where possible, categorise a player's armour description as a variant of existing armour type (armour is not made in factories all alike, there are very many variations). Metal shin guards added to leather armour sounds a lot like "studded leather" to me - it would be simplest to rule it exactly the same for all game mechanics, including cost.
There is very little room amongst the armour types already in the PHB to create something with different mechanics (all AC values, and most variations in other properties are already covered with cost separating otherwise clearly better options). So for most custom descriptions it should just be a matter of granting existing stats to a player's desired appearance or reasoning.