gistfile1.js

// In $.qsa:

var regex = /^([^:]+)([^\s]*)(?:\s)/;

for (var match; (match = query.split(regex)).length > 1; query = match[3])
    context = $(match[1], context).filter(match[2]);

return context;

// In $.fn.filter or wherever the filter logic is (should be $.fn.is):

if (/^:(.+)$/.test(selector) && (RegExp.$1 in $.fn))  
  return this.filter(function(element) {
     return $(element)[RegExp.$1]();
  });

// Then add:
$.fn.visible = function() {
   ...
}

$.fn.hidden = function() {
   return ! this.visible;
}

// and so on...

How do you insert the first part into $.qsa? Looking at it line 104 here, it already returns something else, so I'm not sure where to put it.

It's more complicated than that - the filter function uses $.qsa so the code above will cause an infinite loop. It's mainly a proof a concept rather than actual code. In my humble opinion - the selector processing should go into the $.fn.is function, which would make the filter code more like:

[].filter.call(this, function(element) { return $(element).is(selector); });