This question is inspired by how the Vanity Gen pool operates. Basically there are two sets of keys and addresses: one that is stored at the Wallet hosting provider, the other set is derived from a Brain wallet.
In order to sign the transaction, the keys are combined by adding the two keys (as hex numbers) together (Pubkey + Pubkey) and (PrivKey + PrivKey). Although I think there is an alternate implementation that multiplies the keys.
My question is, has anyone explored the concept of using a Brain Wallet in conjunction with a hosted wallet for additional security?