I hadn't intended to post on this subject until after my return, but here goes!
I too am coeliac (as is my mother) - I have travelled very successfully in Latin America and she has travelled in Northern Spain, again without problems. She recommends living on tortillas - as she speaks no spanish. I have taught myself menu Spanish. I also use "restaurant cards" - see this site:
http://www.celiactravel.com/restaurant-cards.html I print out a couple of sheets of A4 and cut them up into little handouts and actually give one to waiters in restaurants so that they can take them into the kitchen. Used together with a smile and a heartfelt "muchas gracias" they have got me through Chile, Argentina, and Mexico with some absolutely delicious local food, and my enduring gratitude to a bunch of lovely strangers who helped me.
I think it would be perfectly possible to be a (well fed) tourist on the Way, but I'm not convinced that one can be a pilgrim - a tourist demands, a pilgrim is grateful. Just how will I cope with communal meals, a big bowl of pasta, cooked with love, in an Albergue? Add to the mix the hunger (and the desperate need for carbs) that results from a 25 km hike, in a tiny village with no shops, and all the personal "baggage" around the fear of food (which should nurture) being toxic. I think I may learn a lot about myself on this trip.
From my research so far - tortillas are good, safe and widely available (thanks mum!). Skimmed chocolate milk performed better than sports performance drinks for athletes' recovery in a recent research project - so that's breakfast most mornings. Buy it the night before and sip it on the road. (300 calories of protein and carbs). Also, my plan is to do my walking up to about 2pm, have a good lunch as my main meal, either picnic from a shop (plenty of protein from cheese and sausage, carbs from tomatoes and fruit including bananas) or a meal in a cafe with the help of my cards. Also, I can snack on dried fruit and nuts, drink UHT milk if I get hungry and keep a sachet of instant potato in my rucksack for emergency fuel. In this way I hope that even if I can only pick at the salad and drink the vino, I can join in the community of a communal meal or set pilgrim meal in the evenings.
I have also discovered that the paradors pride themselves on having a coeliac menu. Expensive, but perhaps a way to persuade oneself to eat another week of instant potato with the promise of a good meal once a week.
I'll let you know how I get on.
Fiona.