A brain wallet is pretty much useless if the Blockchain is declared illegal information. That's even the most likely scenario of a goverment crackdown - CP links encoded into it ring a bell?
Blockchain stored in nations where it's not illegal, everyone else uses it the same way Bitcoin Wallet for Android does it, where it only requests balances and relevant info for the addresses it's keeping track of. No need to keep copies of the illegal information. Done.
Besides, it would be pretty baseless to make something like an accounting ledger illegal.