I've always been able to get 11 A/K bulkhead on the A330 KIX-SIN-KIX by calling months in advance, with the caveat that these are basinet seats and we might have to give them up with a couple with a baby needed them. Received email confirmation of the seats within a few minutes of the call, and we have never lost the seats.
For our next trip KIX-SIN in on an old-style A380 and the 11 D/F pair are blocked as basinet seats, but I was able to reserve the second bulkhead 18 D/F pair online as they are not basinet sets, so I have not bothered calling to ask for 11 D/F. SIN-KIX is on the new 787-10 and I had no trouble reserving the 11 D/F bulkhead pair online, since they are not basinet seats.
So although limited to this one route, in my experience the only times the J bulkhead seats are blocked is if they are bassinet seats. And I'm not even a member of KrisFlyer; all our flights on SQ have been *A awards booked with UA miles.