Yeah I did this when I had 3x 80mm and 1x 70mm going on a xenon. I had one switch that switched all the fans from 12v to 9v (ground or 3.3v switched off) and I had another switch that switched 12v for all 4 fans with the 5v standby, and it did slowly cool the console down after shutdown.
Ive got a 100cfm air purifier pushing air into my halo3 zephyr's cut open metal cage now, but for some reason it was maxing out the fans after I watched a movie and was doing that, strange I never even messed with the xclamps they are stock.