I'm actually aware of several plastics residents that went from GS to integrated (at least 2 maybe 3+) and I think it happened when the program added another spot to several years, and I think maybe one resident quit which opened another spot. How these lucky people got in is beyond me, but they were at some pretty good GS programs and seemed like really good applicants. All planned on doing a plastics fellowship later on, but jumped at the chance to transfer over. From what I understand, there are exponentially more people who try to apply for these openings than there are spots each year.
Best advice I can give is try for a top gen surg program, work your butt off, publish plastics research, network, and have a lot of luck