I am thinking about riding at least some portion of Stage 15 into Verbier before the riders come through.

What is the best time to do this on race day? Before or after the caravan? How far in front of the leaders do they clear the course?

Thanks!!

You probably won't be able to ride on the road once the caravan is through - you will probably be stopped by the police. Recommend you aim to ride at least 4 hours earlier than the race comes through but it would not surprise me if the final climb is closed even sooner than this.

http://letourenvalais.ch/ for all the details you could ever need.
I myself will be somewhere on the climb on Sunday, a pied sadly, not avec velo.