This is something I've been wondering. Let's say an enemy was wearing power armor, but not a helmet. Does the DT from the armor somehow magically apply to his head as well? Or does it have a separate DT depending on how well protected the head is?

I have no idea, but here is my guess: Yes the head does magically become protected because that's the way it worked in FO3. And FO 1 and 2, come to think of it.

IIRC, amour is protection wise applied to the head and to the body separately so if you shoot someone in the head with no helmet then yes, the body armour will not protect them. Shooting someone in the arm or leg with a sleeveless or legless armour will get protection though

Feel free to correct me if I'm wrong though

I hope it doesn't work like this in hardcoe.

It should be different depending on the body part. Since you tell the armor what is protecting when you make it in the GECK. I think they would put it in and make that change so its body part specific. That would make sense. Plus in the one video of the guy in that town with BPG guys. He aims at a guys chest and says "notice the shield, this means my weapon wouldnt hurt him as its too weak" and when he VATS aims at other parts its not there. So I think its part specific.

No if you look at the walkthrough of primm vid you can see him shooting a powder ganger in the head and even tho its not wearing dt armor on its head hes still getting a red shield icon about dt. Also you can see your char only has a overall dt stat not one for each body part.

I'd actually like DT to be spread to all bodyparts even if it's not protecting them.
Why?
Because no DT on head + VATS = WIN! = Too easy.