Thanks for the replies. Sounds like sticking to the original route is probably the best option, taking appropriate care, of course. I realise the organisers can't advocate ignoring road signs and I wouldn't want them to.
The link I gave earlier (which was official, but aimed at taxi drivers only) stated that the "one-way outbound" measures would be applied after 2pm. Since this is at 250km I will definitely be reaching there after that time!