There are 2 options - the sneaky one and the correct one.
The 'correct' one is that you can book 355 days in advance of your RETURN date. (It is actually easy to work this out for yourself. Today is 10 December, and if you look at ba.com you will see that the latest date you can book a reward flight for is 1 December 2011.)
The 'sneaky' one, to guarantee seats, is to book 354 days in advance of your OUTBOUND date, with a FAKE return date of the next day. You then ring up 2-3 weeks later when your return flights become available and move them, for a small fee.
So ... flights for 27/12/11 should be available c 7 January 2011. You could book the outbound for 27/12 and the return from Narita for 28/12. This lock-in your outbound seats and means no-one else will take them. You then ring up BA a couple of weeks later (depending on your length of stay) and move the return.
PS. Beijing is very cold in December!