Surely, there would be no issue adding cover art, updating your game page or adding feelies. I had no idea that feelies would even be included in the ratings, otherwise I could have added some. I've actually got a map and a walkthrough, but I don't like providing those, as the temptation to use them is too strong for some people and they miss the challenge of exploring the game and solving the puzzles.
Viewing post in Rule 12/updates clarifications (cover art)
My guess is no, too, but I figured I'd make sure. It's low priority for me. And sometimes asking these sorts of questions helps give me an idea of something to write!
BTW, I think it was D'Arkun from '21 IFComp that had the idea of including a password-protected PDF map, where the password was revealed once the player was 20% through the game. That's one idea for reasonable obfuscation that isn't impossible.
As for walkthroughs, one way to do things might be to use ROT-13 for one sentence, ROT-14 for the next and so on, cycling through ROT-25 and ROT-1. That would help people not reveal too many hints at once. I'd think a Python script wouldn't be too hard. For deciphering it, ROT13.com would do well, since it only handles one ROT-X at a time.
(ETA: I'm sure I saw 3 works with no cover art, and now there are 2, and I don't think any have been removed from the comp. So as long as it isn't an "I didn't see how to add this" thing, it's okay, and I certainly have no problem with competitors adding cover art, because that sort of thing motivates me to play a game. Still, it's nice so people have one less thing to worry about for next year.)