Add an 'addParameters' method to Prototype's Ajax.Request, allowing eg onCreate() callbacks to append parameters to requests.

gistfile1.js

Ajax.Request.addMethods({

	/**
	 * Add the values given in the argument hash to an existing Ajax.Request.
	 * This method operates by extending the existing 'parameters' property
	 * on the request, then re-encoding that hash and either updating the
	 * generated URL (for GET requests) or simply setting the 'postBody'
	 * option on the request (for POST and other requests).
	 *
	 * @param parameters	A hash of new parameters to extend the request with.
	 */
	addParameters: function(parameters) {

		// Add new parameters to the parameters hash.
		Object.extend(this.parameters, parameters);

		// Append to querystring if a GET request, otherwise set as postBody option param.
		if(this.method === 'get') {
			// Extract current query parameters if they exist and extend them with new parameters.
			var params = Object.extend(this.url.include('?') ? this.url.toQueryParams() : {}, this.parameters);

			// Trim any existing query parameters off the end of the URL.
			this.url.include('?') && (this.url = this.url.slice(0, this.url.indexOf('?')));

			// Add the new, encoded parameters.
			this.url += '?' + Object.toQueryString(params);

		} else {

			// Just encode and add the new parameters hash to the postBody.
			this.options.postBody = Object.toQueryString(this.parameters);

		}
		
	}

});