I can't catch that before it triggers a "call" to Apple to very the code and do something else? I'm thinking of rerouting to a feature inside my app that's special to users of that code.
Users redeem codes from inside the App Store app on their phone.
You can generate codes to give you app away for free, or to grant a free in-app purchase, but that is all. There's no callbacks from the app store to your app to intercept.
Possibly you could do what you want with QR codes and launch URLs, or by doing camera+OCR stuff yourself inside your app.
This all sounds like it was done without duplicating the font itself (aside from installing it, which does make a copy in one of the Library/Fonts folders, but afaik that isn't been considered duplication for copyright purposes). And it could be done easily enough without even making that copy (a symlink would likely do it).