Can Garnish be extended to allow plugins to use custom keyboard shortcuts?
I'm not sure if there's an "official" way yet but I've looked through the source a little bit and I think the most Craft-like way I would do it, following plugin conventions, is to extend Garnish.Base with a new object that attaches the listener. 

(function($) {

        var PluginName_KeyCapture = Garnish.Base.extend({

            init: function()
            {
                this.addListener(Garnish.$doc, 'keydown', function(ev)
                {
                    if ((ev.metaKey || ev.ctrlKey) &amp;&amp; ev.keyCode == 66)
                    {
                        ev.preventDefault();
                        console.log('You hit CTRL + B! Good work.');
                    }
                });
            }
        });

        Craft.PluginName_KeyCapture = new PluginName_KeyCapture();
    })(jQuery);