I don't know why people keep saying that simple macro's like this aren't trackable, because they're more than trackable.
In the case of a separate key from your default 4 keys being used to press big Taiko notes:Poll the keyboard for any inputs that are not the 4 keys that the player has set, track the offset in the song that they were hit at as well. If the player is repeatedly hitting that key(s) for big don/kat's, then they're clearly using a macro for that key.
No matter what, all keyboard input is trackable (even if its a special macro key), so this would be easily trackable in the above case.
Example: The player is playing a stream consisting entirely of big Don's, the player has the A and S keys set to hit both the z and x (little don keys) at the same time. Considering only the first 4 Don's, we would have an output that looks like this:
Azx Szx Azx Szx
If the player is pressing the A and S keys on time with the big Don's, then they are more than likely using a macro.
In the case of setting the left don/kat key to hit the right don/kat key at the same time and vice versa...Do I even need to explain this one? The player will end up ONLY hitting big Don/Kat's, considering that the mouse doesn't hit hit finishers with a single click anymore, there could easily be a check to see if the player has only hit big Don/Kat's during a song.
..But karterfreak! What if they actually did play the song that way legitimately?
Still simple. There's a very small delay in when a key is pressed before the macro occurs. Therefore if a player is alternating... lets say z/x keys for small don's, they will end up with a key output looking similar to this (uses the same scenario as the previous example)
zx xz zx xz
Ergo, considering a real player will generally NOT be alternating the first key hit when hitting two keys at the same time for a big Don, this can be used as a check against players who hit all big Don's/Kat's through a song.