Some enemies depend heavily on an object they wear or carry to be an effective combatant and so it may be advantageous to attack and destroy the object first, but I can't tell whether this is allowed.
Can I attack an object that is being worn or carried by a creature during combat?
For example:
Official 5e rules designer Jeremy Crawford responded to a related series of questions on Twitter:
When the rules for making an attack or casting a spell talk about targeting an object, is it implied that it's an unattended object, or it possible to target someone's armor/shield directly with an attack in an attempt to damage/break it?
If a game effect lets you target an object, the text of that effect tells you if worn/carried objects are prohibited. The rules don't assume that "object" means "object not currently worn or carried by anyone."
Is this balanced, in practice? When facing someone in Plate with a Shield (AC 21) mightn't it trivialize the fight to target and destroy their armor (AC19, 10hp)? Negating worn armor for the low cost of one weapon attack seems incredibly powerful.
You'll be happy to know that few things in the game do this.
The base rules for damaging objects don't specify unattended, so any character or monster can do it with a basic attack. Am I missing something?
Those rules are entirely in the DM's hands. Using those rules, the DM is encouraged to rule that certain types of attacks/damage do nothing to certain objects. Those rules don't provide a foolproof way to wreck plate.
So in short, this is definitely possible by the rules.
If you're attacking something like a book that the enemy has in their hands or strapped to the outside of their pack (and it's not magically protected), your attack might be more effective, for instance, than if you're trying to break their plate armor.
The DM might give you disadvantage on attacking something small that's hard to target, and they might determine that a damage threshold applies to something that's well-reinforced and hard to damage or say that certain objects are immune to certain damage types.
One of the main pitfalls of combat mechanics that target specific parts or elements of the enemy is that if the PC's can do it then NPC's and Monsters can do it too. Is everyone in your campaign willing to play in a world where their weapons, armor and spellbooks are constantly being destroyed?
I think it's worth investigating the melee combat aspect of this question in the context of called shots, about which many questions have been asked and thoroughly answered on this site. I suggest you run a site search for called shots [dnd-5e] to find all the called shots discussions tagged as dnd-5e.
I don't want to restate what has already been stated on this site numerous times, so here's a link to a great discussion aiming at specific body parts. The top answer by @christutty nails some of the main pitfalls of allowing called shots, and the other answers also cover important aspects of allowing called shots.
In summary:
All these issues are as relevant to this question as they are to any called shot discussion.
There are certain spells that target objects and, unless they state otherwise, they can be used on objects being worn or held by another creature. One such spell that would be very useful in this situation is Chain Lightning (PHB 221). It would be up to your GM to determine what the DEX +/- for Morlock's spell component pouch would be, as well as potential bonuses based on being under partial or full cover behind your enemy's body. When targeting worn objects in combat I think these kinds of issues need to be resolved.
As a reference PHB 196 has a section on Cover. Basically:
I don't know of any RAW that discuss the natural DEX of a worn object, but I would call it as the same as the wearer.
Object HP, AC and other stats and considerations may also need to be resolved before a spell attack on a worn object. For example, you may also need to consider whether a defensive spell cast by the target also protects the object you are targeting on their person.
