Gmail extension/gadget API - how to add a button to the compose toolbar?

The Gmail Labs have special permissions because they are written by Google Employees, unfortunately we mortals don't have such power. There is a way around it of course and you've correctly pointed out that it is to make a Chrome Extension or a UserScript. If you choose to do a Chrome Extension it will just be a wrapper for a UserScript anyway

You will have to create and inject the button programmatically. This will involve quite a bit of scouring the Gmail source code (spoiler: it's ugly).

Without more details about what you want to do, I won't be able to provide much more help but I can help you with one problem right away. You have to make your script wait until the Gmail loading process is done which is a bit of a challenge. This is the solution I'm currently using in Minimalist:

function bootstrap() {    target = document.querySelectorAll('.vt:not(.SFzvCe)');    if (document.querySelectorAll('html.xiu1Fc, html.aao')[0] == null) {        return;    }    if (target.length > 0) {        // loaded, do stuff    } else {        window.setTimeout(bootstrap, 200);    }}window.addEventListener('DOMSubtreeModified', bootstrap);

That version waits for the chat to fully load. Let me know if you have any other questions: @anstosa