This will have to add a global GUI, won't it? If yes, you somehow have to make sure that it does not interfere with anything else touching the global GUI files.
However, maybe this can even be done with the GUI being part of the age, only visible in that age. Then it would be cleanly separated from the actual "global" GUI, but could still be loaded whenever you link there so that it acts like a global GUI. Cyan never did this though, the Jalak GUI is global.