Not sure about the towing capacity of a convertible but glad i'm not the only one out there to have thought of it.I've been toying with putting a "hidden" hitch on mine at some point to do pretty much what you mentioned,however was afraid of the backlash :p .My convertible is a full framed convertible though.My 69' bird coupe had a class three hitch on it and i towed my 15' 1/2 foot boat/trailer on 8 inch rims and would do 100mph and a lot of times i forgot i was towing it Had no problems with it on a coupe. My 69's was welded onto the frame. Problem is if you need to drop your gas tank.Something to also take into consideration unless you have a bolt on style hitch.Keep in mind the structural integrity of a coupe vs. a convertible.Possibly a set of frame connectors would be of help but it is something you should very seriously consider before towing anything of any weight before you do so.My two cents

David