How exactly do custom face mods function with NWN campaigns/modules? Because I can't seem to work out how to get them to work correctly in most modules (which sucks, because purdy faces want).

I don't recall offhand what the name of the mod I grabbed for NWN1 was, I just remember having the same issue. In NWN2 I'm trying to use "50 New Faces of Faerun".

My lovely, pretty face works in the "A Fairy Tale" campaign, but not "Storms of Zehir" or "Tears of Ilmarid", for instance. I've tried poking about in the toolset a little, but can't work out what is different that the latter two are blocking the custom head, but the first isn't. I'm honestly not sure why this is even a thing when running the modules in single player.

I've also tried downloading the .hak for servers and putting it in various places (both hak folders, both override folders variously) to no avail. I tried to add it as one of the .haks for the first Tears of Ilmarid module, also to no avail.

If I have to go through and add the mod as a resource to every module of every campaign I have and get in order to make the face display correctly, I will, but first I need to know how to actually do that... :/

... I'm going to give you the solution points on the grounds of "I'm an idiot" while I go bash my head repeatedly into a wall. Turns out I was attributing the head to the wrong file - it actually came out of Fairys.hak (I feel deep shame admitting my stupidity and wasting your time).

Still, it's a damn nice head, so I'm still going to add it to all the modules so I can use it

>.>

<.<

Consequently, during the digging around I've also noticed that Ilmarid adds buttloads of awesome hairstyles so now I want to add that too XD Don't suppose there's any way to get those into the override instead of manually adding?

No need to add them to your modules.
If you like for example head 67
then copy
P_HHF_Head67.MDB
P_HHF_Head67.N.dds
P_HHF_Head67.dds
P_HHF_Head67.T.dds

into your Documents\Neverwinter Nights 2\override.
If the module your playing has a human female 67, then rename those four files to 167 or a number not in use by the modules hak file. Make a new folder on you desktop & call it Hair.
Open the hak file in the toolset erf editor.
Hold Ctrl & select the .dds, .MDB, .n.dds, & .t.dds of the hair you want & click export.
Click on your new hair directory
Then move your hair directory to override.

For NWN2 you can use sub folders in your override folder to keep things neat.
For NWN1 everything has to go in override.