He spent a day on set...
Rogue One director Gareth Edwards mentioned this on a Celebration Europe panel, which CinemaBlend reports on:
During Star Wars Celebration in London, our own Sean O'Connell was in the room when Rogue One director Gareth Edwards recounted a Monday when George Lucas visited the set, and it turned out to be a weird experience for everyone around that day. He said:
I was told on Friday and spent the whole weekend crapping myself. I challenge anyone to have a more surreal experience than showing a Star Wars film to George Lucas. He'd criticize things, but then we'd realize he was joking. But we'd all be having heart attacks.
...But his creative input was minimal at best
In a 2016 interview with TotalFilm magazine, reported by GamesRadar, Edwards noted that Lucas' approval on certain things was quite important to the creative staff, and influenced some decisions on ensuring that certain props were included in the film:
[W]hen George Lucas says he likes a certain helmet lying around set, it's not surprising that Edwards felt the certain urge to put it in the film.
It was clear from the beginning to Edwards that "whatever [Lucas] liked was pretty much guaranteed to be in the film," as you would expect when the creator of a franchise which has gripped the imaginations of millions takes a tour around the Rogue One design department.
And he got at least one thing (a helmet) into the film:
Edwards says that sitting in the department was "a helmet we loved but didn't know where to put it; there wasn't really an opportunity to use it". But when Lucas was given a tour of the design department, the father of Star Wars "walked past it and went 'That's cool, I like that'".
Unsurprisingly that led to a renewed determination to put the aforementioned mysterious helmet in the film, as the offhand comment by Lucas spurred Edwards into action, and at the mention of the helmet they "looked at each other like, 'OK, we have to put that in the film!'".
But everything was already made, and Lucas had no involvement in the development of any of the characters, sets, or props, and the team was under no obligation to take any of his advice; indeed, he reportedly suggested (possibly jokingly) that they take more advantage of computer-generation, which they opted not to do.
