You can already do this to an extent with Bitcoin. After having a couple of friends be cavalier with the samples I gave them, when I gifted my brother some, I gave him a paper wallet to which I retained a copy of the private key. If he is interested enough to transfer the coins out of it, he can. If he loses it, I can restore it for him. This is not good for real-world usage, of course but you either lock in the transfer or you don't in my eyes (ignoring multi-key stuff for a second).

You can already do this to an extent with Bitcoin. After having a couple of friends be cavalier with the samples I gave them, when I gifted my brother some, I gave him a paper wallet to which I retained a copy of the private key. If he is interested enough to transfer the coins out of it, he can. If he loses it, I can restore it for him. This is not good for real-world usage, of course but you either lock in the transfer or you don't in my eyes.