I get the part about the Customs office being far away...but I think some of the confusion comes from the use of the form 4457.
The 4457 is not to get the firearms out of the country...it's to get them back in when you come home.
You don't need Customs at your local airport because Customs doesn't need to see the firearms at the time you fly out...only when you return and clear Customs for rentry and that will not be done at your local airport.. When you come back, the firearms and serial #'s need to match what's on the form, or you'll have trouble.
So, If you were able to get the 4457 filled out in advance, the system actually works smooth. It took me about 10 extra minutes to clear customs when I came home through New York with a pistol I purchased in Europe....because I had all the paperwork filled out in advance.
I've done this exact same process and it worked flawlessly. In fact, I was shocked when nothing went wrong.