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?
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Combat in D&D has never (except for options) had hit locations and armour class is an abstraction for how well protected you are overall.
You can wear shin guards if you want - they look cool but they won't make one jot of difference to your AC.
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:
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.