Actually, I don't think you would necessarily need to put any sort of mechanics with it. It could just exist as an in game justification for how replacements PCs are typically introduced in the game. You know, how players joke about their characters having a special PC identification card? And there always seems to be PCs of appropriate level waiting in the wings? Well, now, BANG! Now you've got a fun and interesting in character reason for it. Adds a new twist on character death and the ramifications.
As for existing game resources that could provide flavor to support this, I'm not very familiar with 4e, but I understand that devas are reincarnated over a long time, right? What if something caused them to have to merge with living mortals instead?
In 3e, IIRC, there was an Oriental Adventures spell (might have been in the Rokugan book), that took the spirit of a departed character and shunted in into a relation or close friend of the departed character and brought them into the game. This was because raising the dead was seen as unnatural in the game world. You could always take the concept and apply it here.
Finally, if you really wanted a mechanical take on things in 3e and actually stat out the spirit itself, you could always apply the rules from Ghost Walk. The dead PC's spirit would take the form of a ghost until she could find a new host body (a new PC). If every PC of the same player is inhabited by the same ghost, then the ghost itself could get a separate character sheet.