I searched and I searched and I couldn't find anything on the internet showing the art from the preview book. Even the Boom Studios sight doesn't mention it, except for a picture of someone's homemade Kermit puppet holding a copy of the book so you can see the cover, which is a photograph. But I did find this paragraph:
Waid said that, as with the Pixar line, there is no definite artist attached to "Muppets" at this time. He did mention, however, that "our go-to guy on the Muppets is Roger Langridge, who was doing the Muppets material for Disney Adventures magazine before that was cancelled. He's probably better known right now for his work at Marvel doing some of the 'Marvel Monsters' books last summer,
"His stuff is hysterical. And he lo-o-oves the Muppets."
So, there's some additional information.
On further inspection, it appears that Langridge is the one that drew the Fozzie Bear that everyone is complaining about (myself included). The one where he, for no apparent reason, traded in his trademark scarf for a neckerchief and says "Wakka wakka" instead of "Wokka wokka".