And still I don't know why you're assuming he'd even be listened to if he had tried.
The final project means they would listen.
Think of it this way... if you were a huge Muppet fan and wrote a script and showed it around some Muppet Vets who said, "So and So wouldn't say this! This script needs some rewrites, and I'd do things this way..." you'd gladly take the criticism and ask them for help to shape it into something better. This isn't one of those big studio "we wanna get money" type deals where they force a stupid concept into the film, this would be a suggestion from someone who was their near the beginning that personally knew Jim. The fact they even went to him off the bat shows how much respect they have for him. If Frank suggested anything, they'd gladly pull out a notebook and pen and scribble down anything he would have said, even if their arms were hurting. Why? Because any rational Muppet fan who's making a movie would.
It seems that Jason, Nick, and James were all for getting suggestions and feedback from the Muppet people at Disney and the puppeteers. They wouldn't have had a table read without it. Disney coulda just made the entire film sight unseen, but the Muppet staff wanted to craft something that's essentially a fan fic into an official product. Not once during the process did you hear stories of script problems by the studio that got the staff completely angry over the rewrites and suggestions they were given. This is the internet age. If someone so much gets a stomach bug while eating bad catered food on the set of the movie, we'd know what movie, which star, what they ate, and the name of the food company the second it happened.
So, yeah, it's an assumption, but there's far too much evidence to suggest that they didn't welcome the changes. This wasn't a Cool World or Looney Tunes BIA situation.