This is a solution that might work, though that would mean that Will would have to sell bitcoin to users. He would have to become a broker or exchange. I could be wrong, but I doubt he wants to do that. I can't speak for him though.
Is there another way? Could Damus users have to buy via IAP that Apple could get their percentage? What about using an existing broker and their APIs? Though I bet Apple would still see that as Will selling bitcoin.