Bushmaster is a harsh chemical. For as many stories I have had about it halting vertical growth in like 2 applications, I have also heard several posters on this site say that half strength nuked their plants so bad they were unrecoverable.
Edit: Bushmaster is a limited application product meant to be used to initiate the onset of flowering. Used in hydro you put it in the res for a few days after your done veg nutes but before switching to bloom.
What worries me further is Bushmaster is repeatedly referred to in the literature as to be used only on plants that are not stressed, and your flowering plants are stretched to high heaven. However if your going to use it the company suggests strongly that you use a liquid kelp extract alongside it for the kinetin but remember you want the extract, not pressed.
For your really tall stalks though that are slamming into your ceiling bushmaster is not what you need, you need to consider radical surgery and remove the top 1-2' of plant. What scares me is that they seem super tall like some radical thai that I have grown meaning that they are not nearly finished growing. It might be advantages to cut the top and toss it or try to clone the top idk.
Again this is just a suggestion of what I would do, i could be retarded.
Has stretching been a big problem for you? There are non chemical ways to deal with it but you should evaluate it seriously now.