Just to close the loop on my previous post for anyone interested - I was allowed in the SS. They scanned the physical BP and it declined, even though it clearly said "Invitation Signature." The agent took it to someone more experienced, who asked if I had been rerouted. I had and the original routing was cancelled. The original booking was booked business class (Premium Rouge, not Signature) to PTY. The rebooking seemed to carry the business class designation (ignoring that it was PR, which would have been ineligible). Anyway, they waved me in. It's an unusual situation, but I put it out as a data point in case it benefits anyone down the road.