Storing the keys into the ledger itself, maybe via the OP_RETURN field. People have stored images in the blockchain, so I imagine you can use it as a persistent decentralized filesystem in theory. Wouldn't you just need a way to chunk your desired keys into 80-byte segments? The initial wallet used to create the transaction is simply for bootstrapping, after that as long as you know which transactions are necessary to fetch the encrypted keys, and you retain the means of decryption, you could reconstitute the blockchain-stored keys necessary to sign further transactions. At that point you could even dispose of the bootstrapping wallet.
Kinda like if you kept a key in one of those banks that let you rent deposit boxes, and you stored another key there. Then you'd have a way to use the expected greater robustness of the bank (blockchain) compared to, say, keeping the key under your mattress.
Why not just use the existing hierarchical wallets (BIP32) based on a secret you've got somewhere perhaps stored split up using Shamir's Secret Sharing Scheme?
Bitcoin already has brainwallets, password derived private keys.