gistfile1.js

var uservoiceOptions = {
  /* ... */
};

function _loadUserVoice() {
  var s = document.createElement('script');
  s.setAttribute('type', 'text/javascript');
  s.setAttribute('src', ("https:" == document.location.protocol ? "https://" : "http://") + "cdn.uservoice.com/javascripts/widgets/tab.js");
  document.getElementsByTagName('head')[0].appendChild(s);
}
_loadSuper = window.onload;
window.onload = (typeof window.onload != 'function') ? _loadUserVoice : function() { _loadSuper(); _loadUserVoice(); };

What if there is no script tag on the page?

There should always be a script element in a document... at least the one from within which this snippet is running. The snippet could of course be in event handler attribute, but that seems very unlikely.

Is there a reason you are using window.onload instead of event listeners?

Also:
return (typeof original == 'function'
? function() {
original();
_loadUserVoice();
}
: _loadUserVoice
);

Less repetition.