Noteably a creature with multiattack cannot Grapple or Shove instead of one of its attacks, because these are other types of attacks than those the creature can use multiattack for. However Disarm (from the optional rules from DMG) says that you use a weapon attack (which can be one of the normal attack options of the multiattack) that disarms instead of doing damage. Does it work with Multiattack? I'm interested in both RAW and RAI answers.
Disarm. A creature can use a weapon attack to knock a weapon or another item from a target's grasp. The attacker makes an attack roll contested by the target's Strength (Athletics) check or Dexterity (Acrobatics) check. If the attacker wins the contest, the attack causes no damage or other ill effect, but the defender drops the item.
The attacker has disadvantage on its attack roll if the target is holding the item with two or more hands. The target has advantage on its ability check if it is larger than the attacking creature, or disadvantage if it is smaller.