I think you may use the method explained at the link below. You can also use a Beanshell server to change variable value instead of a file or a web site page. http://flaviocysne.blogspot.com.br/2012/06/how-to-dynamically-increasedecrease.html
The link above show how to manually increase/decrease load using some JMeter components and an external check point. This way, you could increase the load at your will and monitor application server and network metrics to see at what point things start to go wrong.
