It would be exploitable. If it becomes more profitable to stay in powerdown and move the HP from account to account, people will actually do that.
This has been a question since the very beginning and (believe it or not) you are not the first one to make such a suggestion. Honestly, it is kind of stupid, like sun's idea to make the powerdown period shorter than the posting reward window. It makes no sense and doesn't fkn work. (lol)
Justin probably just plans to do exactly what you suggested lol.