What, in your opinion, would happen? Not just any old Noxian, I'm talking along the lines of Cass or Urgot, someone big. Someone like Riven can be discussed too, but since her abandonment of the current state of Noxus is public, her case would likely be atypical.
So, a well known and dedicated Noxian knocks on Jarvan's door, bearing gifts of mead and lance polish, and asks to join or otherwise serve Demacia in exchange for looking over whatever that Noxian may have done against Demacia, in war or the political stage. Which of the following do you think would happen?
1. Jarvan agrees in full. Here are your blue clothes and gold armor.
2. Maybe after you serve a reduced sentence. Justice for all.
3. No chance. Guards!
4. Executed on the spot.
5. I have no opinion, but I want Sailor Moon Lux.