Has anyone bought the season pass during a playthrough of the campaign? As in, not before you started playing? I'm curious whether the single player perks get added to your build even if you buy the pass halfway through a playthrough or if you have to have the pass downloaded before you start.

Definitely thinking of getting the pass (I'm hooked on the multiplayer so am really looking forward to those DLC packs...) and am currently a few chapters into my first Survivalist run. The single player perks would definitely help my survivalist run if I could use them...

That depends on how the game is set up to deliver you those goods. They may just appear in your inventory or there may be a specific point in the story you can access them. Like with Bioshock Infinite one of the early on areas is where you get access to all your pre-order/ season pass goodies. I haven't played TLOU so I couldn't tell you for sure with this game.