Add to what christep said, if CX can't get a boarding pass for BKK-BOM, they will most likely be able to do so at HKG and along with it should come a lounge invitation.

At BKK, you don't need to go out of passport control and stay in the sterile area. They have a foot massage parlor near BA/QF's lounge. (Your transit time, it looks to me, is enough to get out and get a two-hour traditional Thai massage in downtown, though. )