You can have arbitrary contractual arrangements (subject to local law), but the underlying property interests still matter, especially during disputes. I believe in the US a reverse mortgage typically means the mortgager (seller) retains legal title, with the mortgagee (buyer) taking a lien. But I believe an "en viager" is closer to the common law life estate, and in the context of reverse mortgage lookalikes would mean the mortgagee getting fee simple title with the mortgager retaining a life estate. A life estate arrangement was common, I believe, in the US before reverse mortgages became a popular product.