not when the engine is running...
You take a head apart for Port & Polishing - and crushed walnuts isn't a BMW Exclusive, there's lots of different types of media blasting: walnuts, glass beads/particles, sand, plastic particles, ceramics...
For the bore, I would've thought they would use some sort of uniform media blasting (as in the medium used has uniform particles). I remember of a new type of media blasting that used much smaller particulates for much more effective results especially used for the bore to increase the ability to hold oil on the cylinder walls longer, but I forget it's name...