I think that the issue here is that all will be fine until there is a prang, and then the injured party has a huge incentive to spill the beans, or lie outright as it suits him.
If you sell a formal share to somebody, they don't have to sell it back to you afterwards
Unless you also get them to sign a post-dated share transfer back to you, but the last time I spoke to a lawyer about that he said this voids the transaction. But it may depend...