Why isn't it likely? I personally think it would make more sense than that the Catastrophe Sakura saw is to take place in some random world. TRC probably only has some four, five volymes more to go before it all ends, so CLAMP will need to start with Fai's background pretty soon if they aren't to rush through it in just one or two volymes. Besides, I'm quite sure that once the gang (not including Fai, seeing as how he has expressed his support in whatever his Only Princess fancies doing) realised Sakura has left/is planning to leave them, they will hunt her down like a pack of dogs (+ a manjuu bun), ensuring she won't be the only one to know of Fai's past.