Having studied the Tour de France stage schedule a bit further I now think that the stage ending at Alpe d'Huez on July 18th may be easier to get to then the one ending at Mont Ventoux.
It still may be necessary to rent a car I guess. But I read up on getting there at Wikitravel and there is a ski lift from Oz-en-Oisans that is sort of the recommended place to park your car. Is it possible to get from Grenoble (or any other city accessible by train) to Oz-En-Oisans with public transportation?