apologies for cross posting but I think it's gonna get more replies here!

We're trying to go to Bangkok next winter(MFU) but surprise surprise! no availability. There is some availability to Hong Kong so I'm thinking of doing the LHR/HK mfu and use CX for the BKK r/t not stopping in HK.The cheapest way I can find to cover the HK>>Bkk bit is using OneWorlds visit Asia pass but the page I've been looking at is a bit short on info.
eg Do I book the CX legs with BA when booking the LHR >Hk legs? online?
How "flexible" will the CX tix be?(the web site talks about "open date" tix?
Will baggage be checked thro?
etc etc any experiences will help.
Cheers