Head stays on. take out the plugs and manually move each cylinder to TDC, cover all other intake ports and blast while putting a vaccum in it to collect the walnuts and carbon and repeat. Sometimes I use some liquid cleaner at the end to get some of the walnut dust out.
I have done 20+ cars using this method. They all still run with no reports of damage to the turbos. The exhaust has a nice roasted walnut smell afterwards. Makes the shop smell nice! I dont think the walnut shells are hard enough to damage turbine blades. You can smash them up into a fine powder pretty easily and they are combustible. The only risk you run is if you do it wrong with the intake valve open, you can get enough walnuts in there to lock the motor up. I always spin the motor by hand when I am done to make sure that it won't lock up.
I recently did a similar experiment on a n54 bmw (D.I. motor, turbo, dual variable valve timing, ect). Blasted and did a catch can and 20k later the valves were still dirty.