Since you won't be taking them out, this is one way of doing it.
First cut small strips of cardboard about 1/4 wide and stuff them down next to the piston. This will prevent large bits of trash from getting down beside the pistons. Then cover the lifter valley and other pistons so that you don't get junk in them.
Then with the piston at TDC scrape the carbon off with a good razor blade. This will get rid of most of it. Don't worry about scratching the pistons, you won't hurt them *but do be careful about the block deck and cylinder walls!*
Then I use a soft wire wheel on a drill to clean off most of the remainder. You won't get it all but don't worry about it. Use compressed air to blow away the dust and then remove the cardboard strips and blow out next to the rings. Rotate the motor around, wiping the walls with a clean rad until the piston doesn't leave traces of junk behind it.
When you are all done spray a little WD40 on the walls so they aren't dry before re-assembling.