I wonder if it could be programed into Flight Deck Solutions CDU. That would work nicely.
What's that product ? An hardware or a software ? If it's an hardware, it's possible to assign a keyboard macro and send it to the PC as if it were a key ?
As I've said, it's not as if GSX is different from any other 3rd party addon that uses an in-game menu that will require at a certain point to select some menu option, using an hotkey first, and then a number from 1 to 9.
Which means, it doesn't make any sense that each and every 3rd party addon would need to be modified for using it without a keyboard, it's way more sensible to have your hardware adapted in a way that it can replace such keyboard functions, which will then allow you to have EVERY foreseeable 3rd party addon that starts with an hotkey and then display a menu, to work, without any changes to it.
If your hardware cannot do that, another solution might be using a programmable keypad, like this:
http://www.amazon.com/Logitech-G13-Programmable-Gameboard-Display/dp/B001NEK2GE