Yeah as @Sean mentioned I tried to do this here - Attempt at a Proc Macro - Interface Action Blocked
WeakAuras can do what it does as it detects the proc but it doesnt try to then perform an action. While it displays a graphic or plays a sound or makes the screen shake, none of these things are considered an action. They don’t attack a mob or heal a player or choose to perform any action that changes the state of the game world. WeakAuras can cause sea sickness by manipulating the visual of your client if you get it to do enough things but its not impacting the in game world only your eyes and ears.
In that other thread I linked, I programmed a special version of GS-E 1 to detect the proc that makes Pyroblast instant and then change to the pyroblast step. I could detect the proc but when I tried to move to the Pyroblast step I got the Interface Action Blocked error. I could even change the macro icon to be Pyroblast but as soon as I clicked it it stopped.
I have been right down in the core of Blizzards macro code to see what I could and couldn’t get it to do within the rules. For example in GSE2 I’m adding the ability to reset a Macro when swapping targets. I didn’t think I could get it to do that but have found a part of the API within the sandbox that can tell what the target is.