Replying to myself... again....
The current beta key still does not work. Makemkvcon on linux refuses to accept it entered via cmdline or via file, with or without quotes, etc.
I ended up purchasing a personal makemkv key which makemkvcon accepted.
So I can only conclude that the bug lies with either the key or the app digesting the key at this point. /shrug
edit: a word of caution. I was using ARM (automatic ripping machine, not the cpu brand) here, and even though makemkvcon accepted the key, i still got "evaluation expired" notices. Turns out i had to run makemkvcon reg as the "arm" user.
so... make sure you 'reg' with whichever user is invoking makemkvcon.