If your lucky it will be the same Orga. as the Serres team. They took a very pragmatic and sensible approach to such matters. If it can be justified without increasing risk, the rule can be changed during the daily meetings.
I think the jacket requirement may have been carried over from earlier rally's, some of which were up in the Greek mountains in October (Frrrrrrrosty)! You would have been delighted to be wearing a jacket.
As it was, in Serres we rode in shirts.