UPDATE: Called and think I got this sorted out. If you are trying to book BA1/2, this does not work with CHASEBA10 if you are connecting in LCY. Your connections code as BA Cityflyer, and although it is a BA marketed flight, it is treated by the system as a "Partner" airline like AA or Iberia metal and excluded. The login error message is just a red herring.

Mildly disappointing, but was able to get upper deck seats on the 744 instead through LHR, so not a dealbreaker for me. CHASEBA10 works flawlessly with AARP (or standalone) after switching that.