A very lightly loaded UPS will send tremendous harmonics upstream. Some new options on UPS equipment include an active input filter to take the filter off line at low load. I would be interested if a resistive load on the UP, at about 60-80% of the UPS rating would limit the upstream harmonics suffieciently to allow the gen to operate. See the articles below for more info.
http://www.powerware.com/About/technologies/tracy.asp http://www.mgeups.com/techinfo/techpap/articles/0246-e.pdf
