The idea came up before on HN[1], where the upshot is, banks don't do this, because if they sell the loan at all, they sell a bunch at once, and there are game-theoretic reasons not to encourage this kind of deal. But Denmark gives you the right to buy your mortgage back under that formula, meaning you benefit from interest rates going both up and down.[2]

[1] https://news.ycombinator.com/item?id=31189814

[2] https://news.ycombinator.com/item?id=32813598