Is the issue really that the back of the bus is using the J "restroom" ?
It's not about the "restroom".
It's about having loitering people and high traffic etc passing thru the cabin.
For me , I could care less who uses the "Restroom". I think this post uses restroom in incorrect reference.
My belief is that the J cabin be kept free of traffic and people coming in and out and thru the cabin at all times.
Given this is Air
Canada isn't it the washroom?
It's a good comment none the less though, my major peeve on a flight last year (I was in Y but I digress) was the person next to me, middle seat, who got up FIVE times in a 3h flight to go visit J.
I have no idea what she was doing beyond the curtain - which LH actually used, but it was very irritating for me.
I have no problem with the occasional J pax wondering back into PE/Y for a brief chat - I understand sometimes only 1/3 pax gets the upgrade, or ones wife does when the husband gets left in PE...
But there's a difference between J->Y and Y->J; and how often it occurs.
So perhaps this thread should be more about how to block people moving into a higher cabin for any reason. In which case fitting sturdy cabin style secure doors accepting only boarding passes and SEMM cards is surely the only solution.