Does anyone have any idea how I can book a WT+ trip with a YUP/KUP in the middle as one ticket?

I want to book LHR->SFO->LAX->LHR on one PNR with the SFO->LAX as a KUP/YUP and the LAX->LHR on the same day (hence one PNR). AA.com doesn't understand WT+ and BA.com will only do SFO->LAX via LHR. Every other booking engine I can think of either doesn't understand WT+ or complains it isn't available on SFO->LAX and won't look for higher classes.