I would like to suggest that the gameboy player (gamecube) would be emulated and somehow merged within the emulator (QT Gui) to show different borders that you can choose when playing games without borders and have it function similarly to the gameboy player. When some key is pressed, an overlay will appear over the emulator and this is when you would choose which border to pick (while rom is active [similar overlay formula like zsnes's overlay when you switch to the emulator options while playing]).

Would function like:

@6:17

Will overlay similarly while functioning like: