I did it a couple of times on my camino, and sometimes meet extra-scruffy pilgrims who´ve ´slept rough´ the night before. It´s mostly done in midsummer, when the moon is bright and the hardy souls who want to make long distances decide to hike into the night and pass up the hot daytime trekking. Earlier in the year you can sack out in grain fields and under trees, and later, after the hay is cut, you can often find haystacks to sleep on.
Just be sure to take away all the trash you produce, and be aware that roadside trees and picnic areas and nice grassy spots are also very popular spots for pilgrim ´relief stations.´ (Which is to say you do NOT want to lay your sleeping bag down in a field toilet.) And in Galicia, the cows arrive early in the morning, and they are curious about the new visitor asleep on their hay. (another unforgettable camino moment for me...waking up to a snuffling wet cow nose next to my face).
I met one poverty-stricken winter pilg last year who´d spent the night at a road-construction site, asleep inside a stack of concrete drainage pipes. Try it. Just don´t expect a great night´s sleep.
Reb.