What do you want to know about the Bosal detachable hitch?
I'm sorry I'm not going to be of much help but I thought I'd reply all the same just to let you know that I'm not ignoring you. I had a look around the office for samples of a Bosal towbar so that I could analyse their design for possible flaws but to no avail. It's always the same, everything else was there except what I was trying to find.
Having said that, there certainly shouldn't be any slack in a detachable towbar system. If you're not getting any joy from Bosal nor from Honda, I'd go to a reputable dealer specialising in towbar fitting and get an independent expert opinion from him.
ps: I presume that the Bosal is lockable like all the others. Does the problem still exist when the knob is locked and the key withdrawn? Maybe the Bosal system has 2 latching positions, a primary latch and a safety back-up secondary latch. Maybe you were only in the secondary position but if this were the case, the knob should not be lockable with the key.