>Then I want to be able to present the discounted subscription when the user either enters a 'promo code' or tap a universal link from outside of the app.
I believe this would not satisfy this guideline:
3.1.1 In-App Purchase:
If you want to unlock features or functionality within your app, (by way of example: subscriptions, in-game currencies, game levels, access to premium content, or unlocking a full version), you must use in-app purchase. Apps may not use their own mechanisms to unlock content or functionality, such as license keys, augmented reality markers, QR codes, etc.....
You can use the promo codes provided by Apple (in limited numbers) to unlock an IAP and use that unlocked IAP to provide access to the discounted subscription. But you cannot use you own promo code or universal link to gain access to the discounted IAP. The issue is preventing your 'selling' such access outside of the IAP (70/30) system.