The ST Owner's Manual expressly states that the ST is not designed or intended to pull trailers of any sort or any size. This may be language inserted by the Hondal Legal Department to expressly remove them from any liability. In the U.S. at least, any attorney worth their fee would use that language to completely absolve anyone other than the operator of the bike from any level of guilt or financial responsibility in the event of an accident, while towing a trailer with the ST.
Note that Honda also never produced any trailer-related accessories for the ST- once again reinforcing their position that trailers and STs do not mix.
Make of that what you will. I'm not saying do, I'm not saying don't. I'm saying that you should expect / be prepared to take full and sole legal and financial responsibility if ANYTHING goes wrong. Your insurance company's attorneys, among others, will not be your friends and allies.