I would be really supprised official or not that the organiser would get in trouble unless any action they did actually caused a problem.
for example,
if someone turns up, acts like a dick, crashes and kills him self and someone else in the process through no fault of his own, i cant see anyone else being blamed but the driver at fault.
although if the event put attendees or the public in danger through the planned actions, ie maybe stopping a whole bunch of cars in the middle of the road on a blind corner that causes an accident.. well that could be another story, but i think there would be so many loop holes to this that would put blame back onto drivers directly involved for there own actions.
its not like were blocking of roads and using them to race illegally. were just driving from point a to point, people are responsible for them selves...
this is just my 2 cents, and would be very suprised to hear that an organiser had ever been held responsible for anything going down legally in the past where actually activites were legal.