Katzman wrote about 5 episodes or thereabouts that season, didn't he? But yes, far less influence than any other season of Dallas, as he wasn't Producer and didn't direct any episodes.
Katzman and Arthur Bernard Lewis were busy on their own series - OUR FAMILY HONOR - which ABC had picked up after they'd done a successful pilot. That was going on during the dream season. And as Snarky says, Katzman had no creative input on the dream season, "creative consultant" title notwithstanding.
Once it became clear OUR FAMILY HONOR was not going to be picked up after the initial run of episodes, Katzman was clearly looking for other work, and that's when the behind the scenes shenanigans to bring him back to Dallas began.
I don't know what stage negotiations to bring Duffy back started, but it's clear from the record that he wasn't confirmed as returning to the cast until after the dream season had been written and wrapped.
The final episode of the dream season had already been written, filmed and edited before Katzman was appointed as the new executive producer. It was 'in the can' with a different ending written by Dunne & Feigenbaum.
But before it went to air, Katzman shot the shower scene with Duffy and an independent crew (apart from his nephew, Wayne Farlow who was Unit Production Manager) and changed the ending - editing Duffy's scene in.
So it was a literally last minute decision. There was no opportunity to do any foreshadowing that the season was going to turn out to be a dream, because none of the creative team who'd overseen the season knew it was going to happen.