The last 4 times I have needed to clear secutity the Platinum AA card worked for me. Also, I am in SIN now and the Platinum card for AA at the C gates was no problem for me and my 2 companions to get through with UA boarding passes.
Also, UA 897 LAX-NRT was really empty in C Class, is this common for shortly after Christmas? C was full NRT-SIN. Good times in C though