Not as a "who is it ok to strike".

So, kind of like challenging someone to a duel, instead of someone trying to harm someone, they would only be trying to inflict relatively small, and short-lived pain.

Something like , one announces that one intends to strike the other person, allowing them the chance to object, explains that if they cannot agree on whether they may strike the person, that they will not object to being struck in response with a proportional amount, adjusted for different levels of pain tolerance, and will not strike further in response to being struck in response. Then if the person does not object, for example, then they strike once (with not excessive force), and wait to allow the other person to strike them (with proportional force) if they so choose.

There is not currently any social convention like this, but provided that the force used and amount of pain are small enough, and not excessively frequent, and people respond to objections reasonably, it doesn't seem like it would be that bad if there was?

Pushing my fingernail against my other finger causes pain, but it does not cause suffering. When pains are small enough, I think the meaning/interpretation of the pain is more significant than the pain, so the actual pain is mostly irrelevant, except in that it stands for what it does.