Then the opposing team will know and take a screenshot, all rounds where he's done that would be subtracted from the total. There's not really many rules you could break and it not being seen in BL (as far as I'm aware).
I pretty much agree with everyone here though, it's not the best time to host a tournament due to there not being custom servers, a solid ruleset etc. It's unfair to base results off random skirmish matches and factions, it's suppose to be even where both teams can play the same map on the other faction. The good thing about the servers though, is that there's no one in spectator, so they can't join in or anything, though that also means if a player times out, they're pretty much gone for the rest of the match.