I had some spare time tonight so the feature is in the last GitHub commit. Installing that firmware is part of the setup process, so you don’t need to do anything special.
Once you’re up and running, activate the “Control Game Frame with SMS” recipe over on IFTTT and then send the message “brightness 50%” (or “b 50%” for short). Things get a little weird below 10%, with some colors disappearing to complete black, but see what works best for you and your environment. Let me know how it goes!
P.S. You can automate this by using the “Schedule Game Frame Brightness” recipe. Just replace the 0-7 field with a percentage. Call it a hidden feature. 🙂