Going to answer this myself, have a few DPs. I have an itinerary for YVR-YYZ-ZRH-JNB-CPT, stopping at each connection point for long hours.
1. Called central concierge to SDC YVR-YYZ, was told SDC is offered for the entire connecting journey, not a single segment, so I can only take or leave whatever the system offers. Told me ticket not eligible for SDC because of the OAL segments.
2. HUCA-ed. 2nd concierge agent asked what the flight YVR-YYZ I'd like to take, and told me there is a $100 charge suggested by the system. Tried to argue that SDC on domestic business should be free but he mentioned system wants $100 because this is technically an international ticket. Didn't want to argue and just elected to pay the $100 to do the SDC. Agent collected credit card information to process payment, but then errored out and couldn't get it done. He put me on hold (presumably he was calling the help desk), came back saying the same story as 1st agent (SDC is offered on the entire connecting journey as a whole). He tried what the system would offer (presumably the same SDC choice we could see on the check-in page), came back saying nothing is offered.
3. I went to YVR concierge office and requested SDC in person. Same story as 2, was quoted $100 fee but errored out in next step. Concierge called help desk in front of me, was told SDC not eligible because of OAL segments in later flights. I politely pointed out that this shouldn't be the case, as people have done SDC on AC-metal with connecting OAL flights. I suggested that the system might error out due to 4th flight is not under airport control yet (more than 48h from departure). Help desk said to hold as she will check with another department. While we were waiting for the help desk to come back, had a chat with concierge and agreed to process this as an invol-exchange and collect $100 flat fee if help desk cannot get it to work. Help desk came back saying she thinks concierge already got it to work as I'm on the new flight's manifest already. I checked with my PNR on cowtool and see both old/new flights on it, concierge said she can see me on new flight but the ticket was not associated, and they were afraid that it could mess up with coupon state and void my connection flights. Help desk agent checked something else and came back to tell concierge to "cancel customer acceptance", then help desk agent completed the SDC and asked concierge to waive the fee due to "AC pedia". I suspect they used the workaround for SDC with route change, but I'm not 100% sure.