Airports that don't come up on the BA website
 

I am looking to fly KA/CX on KWL-HKG, but when booking the ticket, KWL does not come up as a valid city on the BA website. Do I have to call in this situation or is there a way to do this online?

Call up. I've used BA as a oneworld travel agent of sorts before - the agents on the phone can see and price up all oneworld flights.

If this is for a purely Guilin to Hong Kong service then you won't get a revenue ticket from BA for that service online. If it is for a redemption then just enter KWL in My Executive Club and don't wait for a drop down option. I haven't successfully tested this on KWL, it comes back with a System Not Responding, but that looks to be a temporary outage, but this normally works on similar locations. Otherwise, yes, you need to call.