After seventy years of sewing, I've figured out how to cut the bobbin thread from the upper side.
Pulling the needle thread didn't pull up enough bobbin thread to see, and the stitch was embedded in the coarse fabric, so I couldn't pluck it with the seam ripper to pull the loop from both ends, so I lifted the presser foot long enough to pull the work back a bit, so as to make an extra-long stitch that I could catch with the seam ripper.
Lo and behold, the bobbin thread popped to the surface all by itself.