This may help some, but it is worth noting that human brains are notoriously poor at generating enough entropy for a good brainwallet passphrase.   Bitcoin lore already has too many stories of brainwallets drained by attackers running dictionary attacks.  I've seen discussions here, on reddit, on on blogs that provide proof-of-concept code showing how attackers can maintain a local copy of a blockchain and execute a dictionary attack while entirely offline.  The operating assumption (for Bitcoin) is that these attacks are underway and increasingly sophisticated.  

Presumably, the same kind of attack could be tailored to the Nxt Java client here, particularly once the source code is released.  If the value of Nxt rises, Nxt brainwallets would become a more tempting target for any attackers that have already honed their skills on cracking bitcoin brainwallets in the past.

Unlike Bitcoin, Nxt presently relies solely on brainwallets as the means of user authentication.  There is no "cold storage" alternative offered.  I see this security model as a vulnerability, and as future PR problem hurting mainstream adoption if Nxt coin heists do start happening.   Brainwallets are surprisingly tough to get right for the unsophisticated user.