The article I read about this says the aircraft was in holding to burn off fuel, not dump it. This chart describes which Boeing a/c have dump capabilities. http://www.boeing.com/assets/pdf/commer ... eldump.pdf
The chart basically says a 767-300 does not have the capability, unless the airline modifies the a/c for it.
And, the aircraft problem was a bad generator. Except for the inconvenience and boredom of 4 hours of holding, I bet it was just like any other airplane flight. There would be no reason to strap everyone into their seats for 4 hours.