Is it possible to regain a heart via a costume (e.g. the ghost) while being in Tokyo? I'm unsure because the rules state that you cannot gain hearts in Tokyo with the exception of power cards. But costumes aren't Power cards, are they?
Yes, they are power cards so, yes, you can use them to gain hearts.
From the rules:
Costume cards are a new type of Power card for King of Tokyo...
A Costume card is a Power card and is played, like Keep cards, in front of the player who bought it.