I've had times where the AA RTW desk said that the purchased segment(s) had to be the last segment(s) and a few other times where they have let me pick the flights. It doesn't seem to be a hard rule, or at least not a hard rule that's well-known. However, the split has been about 90% the former and 10% the latter. Continuing to call and try different agents might be the best chance of success if it's important enough for you.