Unfortunately this appears to be a driver issue with the joystick. Windows does not recognize it as a button, and so AHK doesn't either. There may be ways around this, but without knowing what exactly Windows does think the button is, we're in the dark.
The best I could propose is to use one of the many AHK scripts that record and play back the last pressed key (start here https://www.google.ca/search?espv=2&q=autohotkey+last+key&spell=1&sa=X&ved=0CBoQvwUoAGoVChMIiK3R8Mm5yAIVxi6ICh1BfQB2&biw=1440&bih=859).
With one of these scripts, it might be possible to toggle these buttons and have AHK relay what it thinks the key is, after which you can bind it as desired.