I recall a similar incident where a crew were rostered HKG-SIN-BKK-HKG all in one day. Weather was fine ex HKG, however forecast for arrival into SIN was TSRA etc.
When they landed, the flight to BKK was delayed due to heavy rains and waiting for the TS to dissipate. They were airborne approximately an hour later, and on their way to BKK, the Capt. decided he wasn't going to complete the BKK-HKG sector by going into discretion.
The F/O supported the Captain's decision, so upon arrival into BKK, the Capt. informed the gate staff that he wanted all the pax onboard and seated, cargo doors closed by a certain time otherwise he wasn't pushing back.
Sure enough, in the typical relaxed manner of the BKK staff, they boarded the flight leisurely...
When the time given by the Capt. expired, he and the F/O packed their bags, asked the passengers (via the ground staff) to disembark once again, as the crew were done for the day.
I was operating a HKG-BKK flight (overnight) later that afternoon, and whilst over Vietnam, we received an ACARS message from Crew Control that our duty had changed and we were now swapping aircraft to fly back these disrupted pax. No big deal - we landed, and swapped planes....mind you, first time I ever saw the ground staff telling me to hurry up as they were 'running with their heads cut off'.
A few days later I spoke with the Capt. who made his decision not to go into discretion and asked him if he was called to the 3rd Floor for tea and scones - his reply was 'No'. Nobody questioned his decision.
Now, in light of all this, I'll admit, I have been a bit laxed in rereading Vol.1, however I heard from another cohort that Discretion is now something that the Company can ask the Captain to undertake whereas in the past they (The Company) could not 'suggest' it - only the Capt. could.
If this is truly the case, I guess your 'protection' on a recorded line is that you use the words "fatigued" and refuse to continue "in the interest of safety", but be prepared to explain yourself on the 3rd Floor.