In general I wouldn't think that inclusion in a dictionary is reason to exclude a word from hogwash. (In fact inclusion in a dictionary seems like a necessary requirement.) But if it was a former Word of the Day or of other such distinction I would not consider it.

Honestly I am surprised that flivver is considered such an easy word! I'm reasonably well read and had never seen it in print until this weekend, and never once have I heard it uttered in person.

Never fear, we shall find a piglet that needs a bath.