If an explicit keypress sequence becomes unavailable or changes, the script appears to call certificateErrorPageController.proceed(). So here is a bookmarklet that replicates this, add it to the URL field of a bookmark and click the bookmark to run it:

    javascript:(function(){if(window.certificateErrorPageController)window.certificateErrorPageController.proceed();})();

It should work as long as the proceed command remains functional and certificateErrorPageController hasn’t been renamed in non-developer builds.