When are you doing this? I'll take a trip from southern Spain through France and Belgium up home in late March, so maybe I can wave at you somewhere ...
I've circumnavigated the Iberian peninsula many many years ago on a bike and also passed through some of the areas described above on another trip. The Massif Central towards the Pyrenees would also be my recommendation, although some parts of the Normandy and Bretagne were quite memorable. Was on Omaha Beach and wondered about all the veterans until I noticed that the date was June 6.
Anyway, I'm watching this thread to get some ideas for my trip, if you don't mind. Any particular nice gravel roads en route?