A fellow member of the historic costume mailing list passed along this link to a news story about an exhibition of paintings, done by a 17th century northern Italian artist, that depict poor people wearing old, ragged garments that look as though they are made out of denim. Not simply a heavy cotton twill, but "blue jeans" material--a heavy cotton twill dyed with indigo. The anonymous artist has been nicknamed the "Master of the Blue Jeans" by modern scholars. The news article contains pictures of three of the paintings, and they do look as though they are depicting denim garments: a ripped and faded apron; a boy's long jacket; and a woman's skirt.
I think this is an interesting piece of evidence, not just for the early use of denim, but for poor people's clothing in period, and I'm glad I learned about it.