You can try this approach:
The Flags value of 0x2000 would mean "log"
Im not sure if it is already your "Plan B" idea, but still it should work
You can't do it with a sequence call but with a LabVIEW call you can do it.
Here's a link to an idea to add it to the sequence adapter: https://forums.ni.com/t5/NI-TestStand-Idea-Exchange/Allow-Sequence-Adapter-to-expand-containers-in-t... (top kudoed in the TestStand forum)
But in LabVIEW you can use the Step.TS.SData.ViCall.Parms["Parameter Name Here"].AdditionalResult container to change whether or not the parameter is logged.
If you are singling out specific endpoints or clusters you'll have to recurse down to the exact value you want: Step.TS.SData.ViCall.Parms["error out"].ArrayClusterEls["source"].AdditionalResult
Hope this helps,