Hmm, I tried this with RTW 1.2 and everybody in the faction got to be great misers.
Code:
;------------------------------------------
Trigger TestFLTRaits1
    WhenToTest CharacterTurnStart

    Condition IsFactionLeader          

   Affects Miserly  10  Chance  100

;------------------------------------------
Trigger TestFLTRaits2
    WhenToTest CharacterTurnEnd

    Condition not IsFactionLeader          
          and FactionLeaderTrait Miserly < 1

   Affects Miserly  10  Chance  100
At the start of turn 2, everybody was a Miser, although only the Faction Leader should have been one. RTW takes invalid conditions and treats them as "true" or ignores them, take your pick. Either way, FactionLeaderTrait doesn't act like a valid condition.