Does this Base effect, which I gather is hardcoded to monitor addictive ingestible usage, also monitor NPC usage so as to apply the withdrawal effect from too much use?

Almost certainly not, I don't think NPCs even use drugs beyond Stimpaks. Psycho won't actually work for them for example, as it relies on a perk.

Thanks for clarifying that - I had seen posts recently that NPCs could have perks, and went off on a tanget thinking I could do all these cool things - I went back and reread the whole thread.

I don't see a value modifier that you could use in an effect for increased weapon damage like the perk does, to allow you to simulate the effect of psycho on an NPC?

NPCs can have some perks. For example Liberty Prime has the bloody mess perk. Some perks will work on NPCs and some won't.

Nope, he doesn't, he just has the Bloody Mess actor value set to 1. There's no field on NPCs to store perks in.
The 'PerkBloodyMess' effect on his actor effects list is the Ability added when you take the Bloody Mess perk, hence it's been named PerkBloodyMess.

So the end result is that he essentially has the perk, meaning that any perks that add abilities can be added to NPCs.

It's an important distinction, as a lot of the interesting perk functions (weapon accuracy & damage, add items on kill etc.) aren't handled by abilities and thus don't work.