So here is a question, normally when something causes a victim to lose fatigue levels, but the target lacks any fatigue levels to lose, maybe they are a magical creature with the No Fatigue quality. I have always assumed nothing happened. It was just too bad for the attacker and lucky for them.
But what if they are engaged in Certamen? Now most Fatigueless buggers can't participate in Certamen, but The Becoming can get rid of the fatigue levels. So what happens? Does one side just get to lose? Certamen isn't spending of fatigue levels, its losing them.
This is to say nothing of the political consequences for making a mockery of a key method of solving differences between magi. That would come back to bite you if you were too much of a jerk with it.
You couldnt do it if you had completed The Becoming, as you give up the Gift in the last stage and become a creature of Fairy, rather than a Hermetic mage. I would imagine that attempting to participate in Certamen after the first step but before the final step would either fail outright or do physical damage to the participant without fatigue.
I guess the ultimate answer would depend on the means by which you think Fatigue is lost in certamen.
As such, I can see anything from the caster being unable to institute certamen, to suffering real physical injury from it. Of course, it's also perfectly possible that the character can't be fatigued and wins automatically, but I dislike that not only because it strikes me as unfair, but also because it seems fundamentally at odds with how lack of a fatigue score has been presented in both HoH: MC and other Ars Magica supplements.
The thing is I never got any sense that not having a fatigue score meant fatigue loss transferred into other areas. I always got the sense that lost fatigue simply was ignored. After a long days hike, you were untired. If someone attacks with a fatigue draining spell you are immune. If you fall a little short of a spell total you lose nothing. So I suppose that is the first question: What happens when someone tries to drain the fatigue of a fatigue-less creature? The next question is then what is Certamen like?
Well, that's because it doesn't necessarily but there are examples like Deprivation where once a character has no Fatigue left they begin taking wounds so it is at least conceivable that a character with no Fatigue (and remember, a character who initiates "Transforming the Body" canonically has no fatigue levels to spend in combat or on spontaneous magic).
I'm not saying it's the correct solution or even one I'd favour in-game, but I do think there's a case to be made...