I want to up this request, it would be wonderful!
It could be something simple like this; In the "Open With" selection, I could either Launch with selected emulator by pressing the X button or Change selected emulator to the default by using the Square-button.
(Think of the Playstation controller)
EDIT:
Also, I made Issue on bitbucket, check it out; https://bitbucket.org/jasondavidcarr/launchbox/issues/4349/change-default-emulator-for-game-in-big