*facepalm*
Going berserk is not a boon. It's not something you can exploit. I'll say it again, the only reason I brought up and wanted to make rules for it was it is something that can happen, and we have been attempting to close all loopholes.
If you go berserk in a bijuu fight and can no longer plan and form coherent thoughts than you lose, period. There doesn't need to be an auto-loss, it isn't god modding, it's just shooting yourself in the foot. I don't know why anyone would do it, but it is a thing that can happen.
Unless you're fighting literal Genin level people who can't deal with a bijuu than going berserk just kills you if you don't have someone around to suppress the bijuu.
It is a self-fixing problem because anyone strong enough to make a Jinchuriki go over their mastery is going to for sure beat them if they go berserk, and so why would you ever willingly do so against someone you can beat otherwise? I seriously doubt it's ever even going to come up.
Just make it be simply like I said. You ascend one tail a turn until all the tails of your tails are out in which case the bijuu is released, you die (unless you're immune to that sort of thing), and then control of the now released beast goes to a judge/GM. You must rp in a berserk animalisitic state or be called out on god-modding.