I haven't actually fought MMA, and I've only recently gotten into learning pressure points, but I don't see why they couldn't be used in MMA.
One thing we have done in class is hit the inside leg nerve (LR-9?) with the elbows when in guard to try to force the leg into a position to get in half guard. It may not work on the more experienced MMAers.
But, another example: if you are trying to put an armbar on someone and they have the hold their opposing hand defense to prevent their arm from being straightened, could activating L-8 help release the opponents hold on himself increasing the chances that you can pull off the armbar technique?
I haven't tried it, I'm just throwing that out there.
I suppose it depends on how exactly you're performing them. Pain compliance tends not to work too well, due to a mix of the pain tolerance of the fighter and the adrenaline of the fight. So generally rather than attacking or pressuing a point because it will cause pain, your objective should instead be ones that cause set physical reactions, or have to obey certain physical rules.
For example, the escape from the guard you mentioned? We learned something similar, where you place your palms in and under their ribs, and your elbows on the soft spot of their inner thighs, and use this to create pressure and leverage enough to pop the legs open. It may also be neccessary to as it was phrased to me "throw your *** out behind you" in order to make enough pressure. Any pain caused during this is just a bonus, the objective is the physics as it were.
With regards your arm-bar suggestion, I'd need a little clarification on the point you are referring to?
As for striking pressure points, falls into a couple of categories. Striking muscle groups for example can be very effective, eg shin kick his thigh, or elbowing the arm muscles, but it requires alot strikes for the effects to kick in, and on more conditioned or muscled fighters, you sometimes might as well be hitting a brick wall.
Striking points that have a certain physical reaction eg jaw, temple, solar plexus, liver, kidney, spleen, tends to be fairly effective if you get it right, so in its way those could be considered pressure points. Striking at other points however, you're looking for ones that will provoke reactions other than pain, simply because pain is not a reliable weapon. Anything that messes up his breathing, or interferes with his bodies ability to move properly, would be the right way to go I imagine.