Probably going to have a go at the 205km too. Seems a shame to go all that way and not at least try to have a crack at the full thing.
For anyone who didn't get an invitation but still really wants to go, I can't see why you couldn't just ride along with the rest. The course is really well signposted (better than the signposting for normal Italian roads) and there are several opportunities to pick up food and water along the route (Siena, Montalcino, etc). So I can't see why you couldn't just go along anyway.
Johnny