This topic does seem interesting.
As someone pointed out earlier yes a Venator was spotted in the rebels TV show but not what you would have expected. It was one of Tarkin's many keep sakes in his office. As rebels pointed out, Tarkin had a fondness for the clone wars, reason he kept commander Gree's helmet and a model Venator in his office. My guess is that he has a Venator model because of his days in that old outpost with the star destroyer Electrum defending
I guess I went a little off topic for a moment, or at least I think I did. In my opinion it wouldn't be that hard for the rebels to acquire a Venator if they managed to get their hands on one early into the war. Most Venators used by the empire were used as fighter training schools with no escorting ships, so it would be easy picking. And crewing with Sullustans or Mon Calamari wouldn't be too hard would it?