GitHub: click anywhere on the "merge message" (with disabled button) and it'll wait until the button enables and click it for you.

githubDelayedMerge.user.js

(function () {

  function handleLoaded() {
    var butan = document.querySelector('.merge-message .btn[disabled]')
    var checkTimer;

    if (butan) {
      butan.parentElement.addEventListener('click', function handleClick(event) {

        if (checkTimer) {
          // already checking
          return
        }

        checkTimer = setInterval(function () {
          if (butan.attributes.getNamedItem('disabled')) {
            // still disabled
            butan.style.outline = (Math.random() * 10)+ 'px outset orange'

          } else {
            // success -- click the button with our previous event
            butan.style.outline = '2px inset green'

            butan.dispatchEvent(event)
            clearInterval(checkTimer)
          }
        }, 500)

      }, false)
    }
  }

  window.addEventListener('load', handleLoaded)
  document.addEventListener('pjax:success', handleLoaded)

})()

Although we hook on load and pjax:success, there's some additional JS re-rendering the button, so this won't work after new commits are added live, for example.