I have asked the modder Pazyryk in this thread whether it would be possible to work tiles beyond the existing city radius. As I do not have a Windows computer with CiV on it, I am asking that someone else who has it uses the instructions in the thread to test if tile yield importation in its most rudimentary form will work - no importation losses or UI are required at this stage. I hope that tile yield importation will work by this method, and if it does not then it is 99.99% likely to be hardcoded.