Finally was able to test my theory and to my surprise it works!

I can control 4 data pins from a link cable to output a 3.3V signal with the press of a button.

The trick is that link cable connectors can be quite different. The GBA one has 4 pins/cables, but there is a difference between the big/small connector. A GBC link cable has 6 pin outputs and in my case an extra ground connector corresponding to the outer plate.

Either way this is very promising!