JsonReader = Ext.extend(Ext.data.JsonReader, {
    exprToPath: function(expr) {
        var parts = expr.split(".");
        var path = [];
        Ext.each(parts, function(item, index, list) {
            var match = item.match(/([a-zA-Z0-9_]+)\[([^\]]+)\]/);
            if (match)
            {
                path.push(match[1]);
                if (/^\d+$/.test(match[2]))
                    path.push(parseInt(match[2]));
                else
                    path.push(match[2]);
            }
            else
            {
                path.push(item);
            }
        });
        return path;
    },  

    getDataValue: function(o, path) {
        var current = o;
        var rPath = path.slice().reverse();
        while (current && rPath.length > 0)
        {
            var key = rPath.pop();
            if (typeof current[key] !== 'undefined') current = current[key]; else return null;
        }

        return current;
    },

    getJsonAccessor: function(expr) {
        var self = this;
        var path = this.exprToPath(expr);
        return function(obj) {
            return self.getDataValue(obj, path);
        };
    }
});

It’s probably a bit of overkill for what is needed, but it does work well.