lore wise, what would happen to a noxian/demacian defector?

If we can take the word of the Journal of Justice still, (probably can't, it's mostly if not all non-canon now), Demacia was just fine with taking Sivir in during her previous lore when she decided to stop lending her mercenary services to Noxus.

Though given retcons and the reworks both city states seem likely to undergo, I wouldn't call that anything to base a current guess on.

I don't think Noxus would care, because a person so low in their hierarchy would have no effect on their war machine. If they got in their way on the warpath, though, Noxus would probably kill them, but it would be no different from anyone else. If the person left and became a great warrior, then they would probably garner respect from Noxians.

Demacia might be more strict, considering their ridged laws (Everyone must serve in the military). If they defected during their military service, they would probably be considered a criminal and be kill-on-sight. If they did so afterwards, it might not be a big deal if they didn't impede Demacia's goals.

Suspicion, distrust, hostility and maybe even severe interrogation or prejudice.
To be expected of warring nations harboring militants from the other side

Lore explains the Demacian attitude.