Or you can do what Vitalik did with his wallets. He gave half of the privatekey of one of his wallets to his father. I see that as risky.
I didn't know about this but this seems to be really risky. But that's what is trust for and they're direct related so it's no issue which others can just copy this if the father is technically aware of how to do things in keeping wallets safe.
No need to complicate things, all you have to do is what mk4 has already. There's no need to think of conventional method or way of keeping your mnemonic phrases.
But it's you, whatever works for you and whatever the the results of it, you're the one who's responsible for it.