I guess I got lucky this morning around 11:30am EDT. I could see the SIN-JFK was available in J on UA.com (SQ26) but it would not allow me to book. Got the message that availability had changed so I called UA and got a great agent. She said she cannot see it but said she could call IT to see if they could see it and grab it. They apparently forced it in and I got the seat. Now I see that nothing is available!
It's ticketed so hopefully I'll be ok.