As far as I know, it's only non-standard resource icons that are outlawed, because of their possible extra visibility under the fog. The GOTM resource graphics are mapped differently to make it harder to use an unapproved set.
I've seen several players posting Snoopy terrain screenshots in the GOTM forums, so I don't think there's any problem with them. I don't happen to like them myself, but that's a matter of personal preference. I know how I would install them in 1.29 for GOTM, but I don't know the mechanism in a PTW GOTM scenario. It's probably just a case of adding the Snoopy files into the GOTM scenario folder, but leaving the GOTM resources.pcx and resources_shadows.pcx files alone.