User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-6951062-20130314091404/@comment-3146930-20130321215156

It isn't beyond impossible as heroes sent to Elysium are given the choice of rebirth. It is possible that when Perseus was reborn, the gods made a constellation to honor his former life. If he was reborn and went to Elysium again, he could have been reborn as Percy.