Hmm that's an interesting observation Cherbourg but unfortunately in most cases it is true.
That being said, I don't classify IGS, TGS or TSS as Brisbane-based schools. IGS and TGS both offer significant opportunities for country kids to have a go. TGS has plenty of blokes from around the catchment region and Darling Downs/Downlands have had a few representative players over the years. TSS has a huge talent pool of its own.
One more thing. The indigenous program at IGS has been exceptional in recent years and produced a number of quality indigenous players. I think most the GPS schools are becoming more aware of the need to focus attention on these types of programs, I know ACGS, NC and BBC have indigenous programs designed to give a sporting advantage but also have academic requirements. They do however tend to choose the best of the bunch, those with previous representative honours.
Your argument stands true though. I'd argue that once the country kids reach about 15 they are pretty much written off unless they have a fantastic season and get noticed by a ARU or QLD scout. By that age, most of the GPS schools and selectors have a fairly good idea as to who should be next on the agenda. There are plenty of exceptions but my best guess would be that we have also let plenty of potential slip through the cracks.