Backbone uses jQuery/Zepto for AJAX requests, so you can use the functionality available in those libraries.
To add custom headers to all XHR calls made by jQuery, you can use the jQuery.ajaxSend event, which is triggered before every ajax request, and modify the jqXHR it receives as an argument.
Edit based on OP's comments:
Probably the simplest way to modify the sent data is to override the Backbone.sync function. You could wrap the native implementation, and add the required property there:
var nativeSync = Backbone.sync;
Backbone.sync = function (method, model, options) {
//for POST/PUT requests, add access token to the request
if(model && (method === 'create' || method === 'update')) {
var data = _.extend(model.toJSON(), {
access_token: 'token'
});
options.data = JSON.stringify(data);
}
//call the native Backbone.sync implementation
nativeSync(method, model, options);
};