Elastic stretching of the brake cable is easy enough to verify or refute for those of you who have a bike with a length of exposed cable along the top tube. Take up the slack by moderately squeezing the brake lever. Then start plucking the cable like a guitar string as you increase the force on the brake lever. As a guitar player and builder, we know that you cannot increase the pitch of a string without either shortening it or elongating it to increase the tension. So, if the pitch of the brake cable increases as you squeeze the brake lever, the tension of the cable is increasing and the cable is stretching. If the frequency doesn't change, the "stretch" is in the housing, shoes, calipers, etc.
Kevin Ko
Eugene, OR USA