As for B&Bs none of the hostels I know (and I know a few) give breakfast. So hostels are just Bs
. Also, I have never heard of places that accept help of their customers in return for room or something like that.
As for monasteries / convents few of them take guests. But it's really difficult to find a place there. From what I've heard, you have to book like 1 year (or even more) in advance and they usually take people who are going to spend a lot of time in a peaceful place to write a book, study...
The best tips from your favourite hostal in Madrid.
Hostal Chelo at http://www.chelo.com