Set what is sent on shift about ergodox-firmware HOT 1 OPEN

If you're still interested, and you haven't already figured it out. Yup, that sounds like a good way. Alternately, you could change the shift key to set another flag somewhere, if checking the shift turns out to be hard (though, i think there might be a function for checking with a key is pressed or not). Alternately again, you could make shift also do a (mostly transparent) layer shift as well as setting the shift modifier bit. Just off the top of my head. I think your first idea is probably the simplest/best :-)

from ergodox-firmware.