this is a very good question. Everyone´s heading home for Holy Week and taking up all the open spaces in the pensions and hostels. Shops are closing at odd hours, restaurants are keeping odd schedules. Parish priests, usually the people to turn to when you need something and can´t find it anywhere, are running their legs off. You´re right to expect some challenges this week, especially in the pueblínes out there in Extremadura.
All I can say is check first the usual way suggested by your guidebook. Then go to the bar or the old folk´s hangout and ask the people there if there´s a room anywhere in town where you can stay. If you can´t find a bar, find a cop. And if that won´t work, go to the plaza mayor and knock on doors til you find someone who will tell you something. It probably won´t come to that, but never despair. There´s someplace in every town where a pilgrim can find a place to sleep, even if it´s on the sofa... or out in the barn! You´ll be fine.
Expect a good welcome in Fuenterroble de Salvatierra, and at Salamanca too!
Rebekah