The difficult part at least for me is that you need to book an appointment for the final interview and most airports are booking 6 weeks or more in the future. If you have a certain travel date and will be going through one of the airports that offer the interview (there's a list on the GOES website of about 25 or so but most airports that are common points of entry are listed) then you could book your appointment for when you are there. It looks like I am going to end up paying to fly down to Seattle primarily for the interview because I don't know far enough in advance when I'm passing through one of the interview places to book an appointment.
I was told by someone at the SEA office that you can check online to look for cancellations and that's what I'm doing for a scheduled trip in late January. Right now SEA is booking in late March!