Back Stay is not required
My 1984 Hunter 31, with the B&R rig has a backstay, like the one you described. I too have thought about putting in a tensioner, but when reading up on the B&R rig, it was designed not to have a backstay at all! I assume our models used a backstay to keep skeptics buying the new design. I have wondered if it did not help, when the spinakker SNAPS into shape, but I never look at the back stay when that happens. Backstay tensioner's creat tighter headsail luff, and more mainsail leach. The B&R rig is to be pre-set, by adjusting the diamonds, and not be adjusted by a backstay. Look up B&R Rig on the internet for more articles about this rig. Do not adjust the backstay, since this could defeat the diamonds, and cause serious long term problems.Hire a professional tuner, familiar with the B&R rig.