I did my GE interview long before the rush, but IIRC after scheduling the interview on line, my confirmation gave a number for the interview location to call if I needed to reschedule. I had the impression then that if you had an interview scheduled, you could sometimes ask to go in and wait for a free spot without an appointment. Especially if you explain about your upcoming trip (be prepared to bring documents such as the itinerary and ticket receipt to the interview just in case), they might be able to squeeze you in or suggest a good time to come in and wait to see if they can take you without an appointment. I think this only works at the location you've chosen. The GE people are nice and it doesn't hurt to ask.