That's quite weird - when using OT on european routes, it doesn't book me into C, D or Z, but it still says "Class C" on my boarding pass.
Before getting *G I also had a trip in plus to BCN, bookingclass A, where I could enter the contract lounge (no *A shengen lounge in BCN), with this boardingpass, which I assumed was because of the "Class C" text.