Not exactly your issue but a few weeks ago I needed to book flights on LAN with BA miles for LIM to CUZ - the online booking engine had a fit, even though the flights showed available on BA.com so we called up the BAEC to sort it. The first agent couldn't do it, but a supervisor could - keep trying and you should find someone who can help.
It can be hit and miss depending on the agent, but supervisors have forced through bookings that look possible online but cant be completed by the usual methods online.
Best. JR.