With {Path to Exile}, can you target a creature in the graveyard?
No. It is a card in the graveyard, only a creature on the battlefield.
609.2. (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=609.2.): Effects apply only to permanents unless the instruction's text states otherwise or they clearly can apply only to objects in one or more other zones.
Example: An effect that changes all lands into creatures won't alter land cards in players' graveyards. But an effect that says spells cost more to cast will apply only to spells on the stack, since a spell is always on the stack while a player is casting it.
Closest ruling I found.