I designed this board to fit over 2 of the screw posts on the gscartsw_lite, and be connected by an 8 pin header cable (working on an easy way to make this). This will listen to the gscartsw_lite and switch to the input that it is on.
Untested, but I may try to build a few soon.
Github: https://github.com/arithmaldor/TOSLINK-Switch-for-gscartsw_lite/releases/tag/0.1
OSHPark: https://oshpark.com/shared_projects/WhT3B602
Enjoy!