The rules are still in review, not finalised yet.
Actually I think I've found a loophole anyway.
Rule says " blah blah blha, primary house leaders can only be challenge another house leaders"
"subordinates can only challenge other subordinates"
Now as
EVERYONE plainly can tell XO's are "secondary" house leaders and not really subordinates.
Therefore XO's are clearly only challengable by no one.
I like this.
What do you mean "not finalized yet" ?