I dont think that you can get the return ex-CMB bp at BKK.

If you purchased your tickets online, just head to the transit desk and they'll get your boarding pass / lougne invite etc sorted - you may see massive queues at times, but they should clear quite quickly - I have seen a large group waiting in queue impatiently, and then heading en-masse to the immigration desks, twice.

If you bought the ex-CMB ticket from CX's local reservations office and need to sign the charge slip, notify them that you're doing a turnaround and they'll have someone waiting for you at the jetbridge. (last time they told me 'incase you can't find the person, please go to the transfer desk and specifically ask for Cathay staff instead of UL staff who do most of the ground ops for CX)

Oh, and in the latter case, do give them a couple of minutes. Last time i did a turnaround, I was first off the plane and didnt see anyone in the jetbridge, but bumped into the person riding the travelator holding my name-tag-card heading towards the gate, as I made my way to the transit desk.