To make armor sets more interesting and unique, I'd love to be able to add set bonuses to armors, giving the wearer certain perks that they only receive by wearing the complete set. Now, I know that there aren't any examples of such in the vanilla files, so I was wondering if anybody knew if this were possible to add in game? I've been hoping that it would just mean adding a line or two of code to the corresponding armor files, but i doubt it's that easy, if it is currently feasible.
Not directly, though there are some ways to accomplish it. I believe someone did this a while ago by applying different new status effects, and checking if the player has all 4 status effects (inside the status effect script, or maybe a player/tech script).
You can also get the armor which the player is wearing... You need to do this from a script which has access to the player table (a quest script for example)