What SKB said. I have a nice .45 stuck in Kenya because I didn't know about Form 4457 when I brought it from the US. Had no trouble (at the US end) bringing back a shotgun I brought in later for which I had the form. It's just a matter of presenting the guns to Customs outbound and having them stamp the form.
I think I remember that you had this gun built for you. Any chance you could ask the gunmaker to draw up a receipt for you from his records? I bet you could get it back into the country if you did.