Looks like a good idea. I wouldn't buy one but I would like to hear about it from someone who did.
http://www.jegs.com/p/AEM/AEM-Dyno-Shaft-Pro-Series-On-Vehicle-Dynamometer/1725271/10002/-1
http://www.youtube.com/watch?v=M4q_7jI0_5Q#t=319
http://www.jegs.com/p/AEM/AEM-Dyno-Shaft-Pro-Series-On-Vehicle-Dynamometer/1725271/10002/-1
http://www.youtube.com/watch?v=M4q_7jI0_5Q#t=319