After running regular brake ducts at all of the winter tests, McLaren showed up in Melbourne with a gauze fitted onto the brake duct. Knowing that Albert Park is a street circuit in the middle of a park where leafs are all over the track, such a gauze makes sense to prevent any object getting stuck into the duct and preventing decent brake cooling.
Although this certainly is helpful, there is an efficiency penalty as air is slowed down, and as such less molecules will flow into the duct for cooling. This is also why no other team has taken this route, and why McLaren is unlikely to repeat this in future GP's.
Also note that the black tape covering a small part of the inlet was present during some runs on Friday. They are used to either test brake cooling or reduce cooling when for instance ambient temperatures are low.