FWIW, I finally remembered to email Cunard about my question re: OBC when using an FCC. Here's the reply. I think the conditions must have been tightened, because I'm virtually certain I've used FCCs against the types of (now prohibited) fares and received the OBC.
Thank you for your inquiry about the benefits of our onboard sales program. I am sorry to advise that not all of our promotions combine with the onboard sales program due to the discounts that certain promotions provide. In general, the program will not combine with holiday promotions, last-minute fares, or other fares within the final payment period.
You did receive the benefit on your voyage with us this past January as it was booked on a standard public fare.
I apologize for the disappointing answer and wish you both the very best,