If I remember correctly, PP attacks are not allowed in MMA. However, due to their nature, they are at best (as DAT stated) a hit-or-miss weapon under actual combat applications. I would actually think that you would have a much larger chance of winning the lottery than being able to use PP attacks with reliable results in an actual fight; especially against someone who is trained.
Case in point: One thing that I sadistically do to get someone's arm off of me (if they are the average Joe) is shooting a spear hand up into a person's armpit. Usually you can catch a person off guard and cause a consistent enough amount of pain to make them recoil quickly. When I used to roll with one of my associates from my brother's church, I found myself mounted and in the midst of trying to bridge/upa out of it I snuck the spear into place. After the intitial flinch, I had the same arm I used snatched from its safe haven and barred viciously.
The same guy told me about how to get into someone's nerves just enough to make them move to escape, but warned about how in an actual fight it would be a lot better trying to tie up the person on top and sweeping them because it doesn't do me any good to fish for nerve clusters if my head is being bounced off of the canvas from punches. Further, through rolling on a regular basis, if your opponent is conscious enough to fight you for control of your arms, head, or legs (which they will do whether they are skilled or not) it becomes too much of an issue to look for a fine pressure-point area regardless if you're in a good position or not.