I executed the command replaceitem entity @a container.0 snowball 16 constantly with a repeating command block. In creative mode I opened my creative inventory, hovered over an item with my cursor, and pressed my "hotbar slot 1" button (default is 1) twice. Instead of having the item become instantly replaced with the snowballs, the item stays there.
If I press "use item" (right button) while holding the item, unless the item would normally place or spawn something, the snowballs still throw. If the item was a trident or a sword, no breaking sound or animation is made when breaking blocks with it. As soon as I open a container the item gets replaced by snowballs. I believe the item only appears client-side as it is not recognized by commands such as clear or enchant.