Hi alagria- ahhh so you have walked this route before so know how harsh it can indeed be. I don't have details for transport across it, but I am sure that once you reach somewhere like Aumont-Aubrac you will be able to find information if the weather is bad. There are luggage services in the area these days that also convey pilgrims, but in any case, the road distance from Aumont-Aubrac to the lower level of St Chely d'Aubrac is not so far. Perhaps a taxi would be available and not too expensive.
I walked the route mid-April in 2008, and the weather was indeed bleak. There was so much fog on day 2, I couldn't see much at all- and in any case the gite owner advised us to walk the road route as the chemin was so wet from rain the previous day. However, it was a challenge that in hindsight I remember 'fondly'. I re-walked it last year in early May and the weather was superb- close to 30C, and I walked in the sunshine in a t-shirt. The contrast in weather conditions was quite staggering!
I hope it all works out for you, and maybe you will in fact be 'lucky' with the weather.
Margaret