That's a fun question. However, before we can answer that it's important to note which kind of masochism you want Meta Knight to find joy in. I believe that masochism can be divided into categories.
- Physical Masochism (Meta Knight who becomes enlightened to the joys of pain and overstimulation.)
- Humiliation Masochism (Meta Knight who becomes enlightened to the joys of being humiliated and treated like an animal.)
- Submission Masochism (Meta Knight who becomes enlightened to the joys of servitude of someone higher than him.)
There are sub categories within these categories like 'Humiliation Masochism ➡ Being Treated Like a Pet Masochism'. I think for Meta Knight to be enlightened to any form of masochism you'd have to play off of his inherent traits and desires!
I think getting him to become a subservient masochist who loves following orders and being at the whims of another person can be done by playing into his desire to serve and protect as a knight, chivalry too if the person dominating him is a woman. If you can make this into something of a comfort of Meta Knight, a reason to let go of the high level of restrictions and standards he holds himself to, Meta Knight is more likely to submit.
As for physical masochism, I like the idea of Meta Knight being inherently impervious to pain or damage so pain as a sensation would be something alien to him, something exciting. I think if you combined pain with pleasure but slowly increased the amount of pain over pleasure Meta Knight would fold quickly and become addicted to such overstimulating sensations that he wouldn't be used to. It would be like alcohol, at first it's a little strong and even slightly nasty but the more you try it the more you want it.
As for humiliation masochism, that's super easy! You just need to break down his pride and his ego, and most importantly mix in some pleasure with that slow deterioration of his ego and pride. Most importantly, you must take advantage of Meta Knight's complexes like his desire to be seen as cool, manly, and strong! Similar to subservient masochism, if you can make it into something he can lose himself in he's likely to submit!