I have a Cathay membership that I use to log in when booking a flight. I've never collected miles with it; I always credit to my BA Executive Club membership.
I was able to check-in 48 hours before my flight a few days ago.

(I think doing what I do excludes me from the 24-hour post-booking cool-off period though. I suppose I could just add the Asia Miles number when I book and then swap it to the BA one after 24 hours ...)