I understand your idea, but it sadly can't be done. I understand that you are saying, that a option would be available for people to have a "self-defense" mechanism is place when their hitpoints reach a certain spot. For instance, if I have 300 hitpoints and I have my option set for 150 hitpoints and I log off and someone attacks me they will be able to PVP as usual but when my hitpoints get to my target point of 150, my "self-defense" jutsu would kick in (if they have the chakra required).
1) One would have to constantly change their target points, due to low hitpoints in the early levels.
2) After the first defense mechanism is triggered, and if you win, your chakra points would need to be deleted while not being logged on (plenty of coding). In addition, if a second person attacks you, would you still have the same amount of chakra points since it is a new attacker.
3) A lot of coding, just for PVP when the current system is fine (not quite sure why people want to use jutsus in PVP. Perhaps a outside chance to defeat a Daimyo or a really formidable opponent). By using jutsus to defeat a formidable opponent, one wouldn't get any clout or recognition; you had to use jutsus while the other person didn't.
4) I dislike coming up with reasons why an idea won't work, because at least you put thought into coming up with an idea to improve the gameplay for everyone. However, asking for this is like a Kage asking for the ability to use his or her mount, curse seal level 2, and jutsus in PVP against a Daimyo. There's nothing special about that kind of PVP, it's completely flawed.