Currently new selectors cannot be added via the extension API documented in the Sizzle wiki: http://github.com/jeresig/sizzle/wiki

The issue is the

leftMatch

for ( var type in Expr.match ) {
  Expr.match[ type ] = new RegExp( Expr.match[ type ].source + (/(?![^\\[]*\\])(?![^\\(]*\\))/.source) );
  Expr.leftMatch[ type ] = new RegExp( /(^(?:.|\\r|\\n)*?)/.source + Expr.match[ type ].source.replace(/\\\\(\\d+)/g, fescape) );
}

We need this code to be publicly exposed so that we can run it after adding new selectors via the extension API.

I've hacked around this by creating a new method (using Sizzle via jQuery):

Expr = $.expr;
Expr.process = function(type) {
  Expr.match[ type ] = new RegExp( Expr.match[ type ].source + /(?![^\\[]*\\])(?![^\\(]*\\))/.source );
  Expr.leftMatch[ type ] = new RegExp( /(^(?:.|\\r|\\n)*?)/.source + Expr.match[ type ].source.replace(/\\\\(\\d+)/g, function(all, num){
    return "\\\\" + (num - 0 + 1);
  }));
};

which I use like this:

$.expr.match['WIDGET'] = /@((?:[\\w\\u00c0-\\uFFFF\\-]|\\\\.)+)/;

$.expr.preFilter.WIDGET = function(match) {
  return match[1].replace(/\\\\/g, "");
};

$.expr.filter.WIDGET = function(elem, match) {
  return elem.getAttribute('data-widget') === match;
};

$.expr.process('WIDGET');

Sizzle needs to expose the code that fills the leftMatch object via a function that works something like the one I've shown here.