I have a way we can potentially make the use of genjutsu fair in fights with judges, particularly biju fights. We introduce a sort of "buddy system". The whole point of genjutsu is to manipulate your opponent's senses to produce a desired effect. It's almost always used in a subtle and deceptive manner. Outright posting that you attempt to place your opponent in a genjutsu defeats the purpose, and encourages metagaming and insta-negation of genjutsu.
So instead, the way I suggest we do it (which is how I've typically done it on other sites) is to make subtle references toward the potential activation of a genjutsu in one's posts and leave it to the opponent to pick up on it or not. This would work especially well if the environment within which a fight takes place is described before hand, or if emphasis is placed on a particular instance in the course of a fight that could be used to cast a genjutsu.
An example of this would be you making reference to your character waving their hand and giving particular details about it, which isn't something people would normally put a lot of detail into in the middle of a fight.
Another example could be you describing that there is a particular smell of lilac in the area.
Of course if we're talking about non-dojutsu based genjutsu, then it would likely need to be accompanied by a hand seal of some sort just to make it fairer. That could easily be concealed among the hand-signs for another technique as well.
Another thing we could do, particularly for fights with judges is instead of posting the genjutsu they can PM the judge with the jutsu used and a link to the thread. Then it can’t be claimed at a later point that the user was god-modding and had never done the technique.