The question of post-II materials was asked multiple times during my interview day. The admissions officer told us AMC did, in fact, accept post-II materials if it was something not listed/mentioned anywhere else in your application... and that we should send them within 1-2 weeks of our interview. She encouraged us NOT to send something if it was mentioned anywhere else in our app, but she made it clear we could send in post-II materials if they were completely new. I had my doc send it to the regular admissions email, and he told me he got a confirmation email a few days later.
She also pretty explicitly told us not to send thank you letters/emails because it would just get stuffed away into our chart and wouldn’t really go to anyone. I figured that was the case for most schools, but it was nice to actually hear an admissions officer be real and honest about that.