You guys are a great resource. Thank you, sincerely. Having sorted through the responses, done a little more research (including reading BrianJohn's great 2008 thread on the topic) and talked amongst my peers I have a few follow-up items:
1. As near as we can tell there is no reason for the GFP in the 208V distribution panel. We suspect it was poor design. The only existing loads fed off this panel are office areas, gymnasium areas, and a few mechanical room/loading dock areas. The only new loads are for fitness equipment that was not yet on site at the time of the event.
2. The GFP device is an ITI BGFL-259-1200, installed circa 1999. I reviewed the literature, it doesn't come out and say it but FWIW I assume this is a zero sequence system.
3. The existing breakers are Siemens, I assume type BL breakers. Judging by the 1998 drawings most are 20A/1P.
I was leaning towards cranking the settings to the max, but after plotting the GFP with the 20A/1P breaker on a TCC I see how the minimum settings could have caused a trip. Cranking it up to 300A or 400A would put some space between them. I will recommend 400A, say with a brief explanation why I ~think~ it will solve the problem, and add if they are serious about 100% avoidance there are a number of measures they may find worth taking.
So that's the direction I'm headed. Beyond that I have a few more what-if questions...
4. Is it correct to say (assuming correct wiring techniques were used) that whatever caused the GFP to trip was something on the secondary side of the delta-wye transformer, possibly including a failure in the GFP itself? That an event on the primary side would not have passed through to the secondary?
5. And vice versa - if we effectively disabled the 208V GFP could we assume if an event repeated itself on the 208V side that there is virtually no chance it would now trip out the main 480V GFP?
6. What is worst-case scenario if we cranked up the GFP settings to the max? The GFP is for equipment protection, not personnel protection, correct? Is it possible that cranking up the settings would expose personnel to a life-safety hazard that would otherwise be avoided?
7. Do I read correctly that space heaters are often the source of intermittent ground faults? The day in question was not a particularly cold day (mid 30s, when it has been below zero quite a bit this month). But I was told there may have been almost a dozen electric space heaters in use at various workstations.
Thanks again for the excellent insight!!