We already got the ! if you get discovered by enemies, did you forget? :P
Now someone make the cardboard! Mwahaha!
Jokes aside, I'm not sure if it can be done, but I have seen people edit default stuff, so I hope someone will do it for you and since you don't seem to be the only person to want this, I assume someone who wants "normal" sneaking will create this. Not sure if the other mods break online compatibility though, we got barely any information on version mismatch issues except for "don't edit the main module directly through the editor". Not sure about assets and animations here, but my guess would be: If your coop partner doesn't change his/her game the same way, there will be issues.