Iam not sure, as iam not using the BTC option anymore after I had some quite underwhelming experiences with these meals. I know that BTC is widely popular with people flying occasionally with SQ, but honestly most of the Asian options are not designed for being served on 30.000ft and due to that the taste is not to my palate.
Anyway, there is still plenty of time to make your BTC selection as they allow selection >24h before departure. At some point you have to decide whether to cancel your waitlisted booking anyway and then seat selection/BTC should be accessible online.