If there is anyone with the Numpad and the ANSI keyboard that wants to try a custom solution, with the caveat that it is completely untested (I don’t have my 16 yet, and I’m not even ordering a numpad right now), I’ve thrown together a quick “altsync” branch of the Framework QMK firmware here:
It uses numlock and capslock to communicate the state of the alt keys on the main keyboard. There may be timing issues, so I’ve turned on console logging which can be watched with the QMK toolbox.
If you want to try it out with pre-compiled firmware, you can get the two files here (link expires in 2 months). I could also generate it for other keyboard types if desired. That said, anyone who wants to try this should be comfortable with replacing the firmware, as I’m not even sure where to get the default firmware image or how to update it myself yet!