justin2taylor wrote for some reason it's not recognizing my events. The commands are still going to Premiere and skipping my panel. Have you had success with this?

There are several issues here of which you should be aware:

CSInterface.registerKeyEventsInterest Takes a JSON String

According to the documentation (as well as successful tests), you must pass a JSON stringified version of the keyEventsInterest object into the registerKeyEventsInterest function. This would turn your code into the following:

var csInterface = new CSInterface();
var keyEvents = [{
    "keyCode": 90,
    "metaKey": true
}];
// Pass a JSON-ified version.
csInterface.registerKeyEventsInterest(JSON.stringify(keyEvents));

KeyCode Registration is Different Between Mac/Win

The HTML key input events report identical KeyCodes regardless of the host OS. One list of KeyCodes for HTML callbacks can be found here. This is not the case for the registerKeyEventsInterest function, however! Rather annoyingly, KeyCodes sent to Adobe applications are platform-specific. A list of KeyCodes for MAC can be found here. A list of KeyCodes for WIN can be found here (I believe these are identical to the HTML spec). Thus, to register for both Windows and Mac platforms, your code would look like this:

// Is Platform Mac or Win?
var isMac = (navigator.userAgent.indexOf("Mac") != -1);
// Register for interest.
var csInterface = new CSInterface();
var keyEvents = [];
if (isMac)
{
    keyEvents.push({
        "keyCode": 6,   // Z - in hex: 0x06 (or 0x6).
        "metaKey": true // WARNING: This is probably buggy.
        //"ctrlKey": true // <-- SUGGESTION: Use this instead of metaKey.
    });
}
else
{
    keyEvents.push({
        "keyCode": 90,  // Z - in hex: 0x5A.
        "ctrlKey": true // Use CTRL on Windows.
    });
}
// Register with Premiere. Pass a JSON-ified version.
csInterface.registerKeyEventsInterest(JSON.stringify(keyEvents));
// Register with HTML.
$("#editor").on("keyup", function(e){
    e.preventDefault();
    if(e.keyCode == 90) // Z.
    {
        if ((isMac && event.metaKey) || // WARNING: This is probably buggy.
           //((isMac && event.ctrlKey) || // <-- SUGGESTION: Use this instead of metaKey.
            (!isMac && event.ctrlKey))
        {
            undoEdit();
        }
    }
});

NOTE: You can use hexadecimal numbers in your JavaScript for both the HTML event handlers and the registerKeyEventsInterest function.

You May Be Experiencing A Bug

There are actually two possible bugs that are biting you right now.

• Panels Stop Registering Keys Altogether - I've had a rough go of it with getting keyboard input handling to work. The one thing I've yet to try is resetting my computer (or killing all Adobe processes...).
  • EDIT: It turns out this was caused by a bug in my code. I'm leaving it here as a reference in case others encounter a similar issue by trying to use multi-line strings, rather than JSON.stringify-ing an object literal (or equivalent).
• CEP Fails to Register the Command Key on macOS - I've confirmed with the Adobe team that there is a bug with CEP failing to register key interest for the Command key on the mac - it simply fails to so. Apparently this is being tracked in their internal bug tracker as issue CEP-445. In the meantime, I would highly suggest using the ctrlKey rather than the metaKey on macOS as well (as suggested in the example code).

I hope this is helpful.