I don't say, burning plastic bottles is a good idea, but dioxins are not a likely problem.
first: plastic bottles are usually PET - thats Polyethylenterephthalat - neither Cl nor Br involved here.
The group of Dioxins are Polychlorinated dibenzo-p-dioxins and dibenzofurans - so they need the presence of Cl to develop.
second: there ist the dioxin window: thats the temperature range in which dioxins can form. This window is between 300°C and 700°C.
Wood burning temperatures are between (600°C –1300°C), if the rocket stove is actually burning at the upper limit, it should exceed the 700°C border. This means the Dioxin molecules will dissociate.
So in a properly burning rocket stove I see no problem with the development of dioxins.
I'd like to weigh in on the burning plastic. There's a tremendous amount of energy encased in plastic (18-19,000 BTU per pound). The problem in a rocket stove, as I see it, is not nearly enough air. It would have to be force fed. Pennsylvania State U. has designed and the Koreans have built a safe plastic heater. It's worth checking out.
#2 and #4 plastics are basically oil. One of these days we are gonna be mining our landfills.
Just put the cards in their christmas stocking and PRESTO! They get it now! It's like you're the harry potter of permaculture. richsoil.com/cards