After 3 years of study, last year I qualified as a shiatsu practitioner (Masunaga Zen Style). So yes I know lots of points but I'd hardly say I'm an expert at pressure point manipulation as regards to fighting. We were only taught the beneficial and contra-indicatory aspects of each point not the detrimental effects when used with harmful intent.
In theory I would be more wary of a Dim Mak practitioner as that is what they specialise in although there is a lot of contention as to the actual effectiveness when used in real situations. When the adrenaline and endorphins are released the body becomes less sensitive to pain thus reducing the way points hurt. Another thing to bear in mind is that to strike a point accurately can be incredibly difficult. Another thing to consider is that sometimes pressure points just don't hurt some people.
Now the complicated bit...
When using accupoints (pressure points) and the meridians in shiatsu for healing the general idea is as you mentioned "correct the energies of the body". The general TCM (traditional chinese medicine) idea of the cause of illness is said to be due to the disruption of ki (energy) flow. This manifests as pain/illness so therefore in shiatsu we manipulate the meridians and accupoints to encourage the flow of Ki. This is done largely by the intent of the practitioner as they work. For example if you find a ki blockage, you would then massage it or just hold it while you focus your 'intent' through your hands. Your 'intent' causes your own ki to flow down and encourage your patients ki to flow. This isn't an instantaneous process it takes time and practice to get it right and make it effective.
With this in mind you should in theory be able to disrupt your opponants ki by 'intending' harm when you are fighting. Indeed to a certain degree this will already be happening as most people in a fight don't do very well if they don't intend to hurt their opponent when they hit them. So going by what I've already said you would have to practice being able to focus your harmful intent in an energetic way at the precise moment of impact in order to get your ki to disrupt the ki of the person you've hit.
If you subscribe to the Ki/Meridian/Pressure point theories it should be possible, a lot of people don't and scoff at those that do. However having said all that, I do often have fun with people I'm working with during class as there is nothing worse than being in a joint lock or hold and then having a pressure point poked really hard...
