Is it possible to have roughing the passer on a pass thrown backwards from in or behind the neutral zone
SCHSref:
Below is the definition of a passer from the 2019 Rules Book.
ART. 11 . . . A passer is a player who throws a legal
forward pass. He continues to be a passer until the legal forward pass ends or until he moves to participate in the play.
Based on the above definition, the answer to your question is NO.
You may have a personal foul, but it is not Roughing the Passer.