Yes they can and do register within a few weeks before the actual race but the teams have their own formal schedule completed a year ago in order to plan for flights, hotel rooms, etc.
Until they pave and restore Main street they would need mountain bikes to race anyways. Some of the racers I talked to stated they aren't use to the road conditions here. Just because we were the oldest race doesn't mean the racers were obligated to attend. At one time small continuous loops of local races were popular but now long stretches of roadway is now the norm.
Remember they are use to racing in California where most of the riders are from.