I spoke with the folks at Rig Rite who have the Kenyon spars. They tell me that this was common with the old booms, and apparently they've redesigned the boom to prevent this. But you have to buy the new boom.
Looks like I'll be buying the stainless reinforcing plates, switching to the larger clew slug from my old main, and putting on a velcro strap for security. Thanks to all for the advice!
