Well, this is what I think:
-Since littering is forbidden everywhere, no signs would be needed to specifically indicate so. If you live in Yliakum, you know.
-Since littering is forbidden everywhere, you'd obviously know the places where you are allowed to. No sign would be specifically needed.
And I would recommend to enter the ducts and place the items at the bottom of them, since that would happen if you would throw them in. Best would be to contact a GM, but I think that's pushing it. If I have items in surplus, I just give them to some noob (there are always plenty) or dump them in my guild repository.