Beta 4 isn't out yet.
If you have one of the maglx3 builds, this video showcases some things you can do in lunalua to monitor the player's actions and interject (i.e. how you'd make a custom powerup).
https://www.youtube.com/watch?v=yWfv8RkAKSI
Making a powerup then just becomes combining checks and instructions to make the program behave like you want it to.
Lunalua documentation beyond the video's description:
http://wohlsoft.ru/pgewiki/Category:LunaLua_API
Fundamental lua tutorials:
http://lua-users.org/wiki/TutorialDirectory