Clean dirty pistons while still in engine?
Pictures are worth a thousand words for what you are dealing with but try to not invite trouble. Cleaners inherently dry out everything and if you are rotating the motor to get a better piston position for cleaning don't be bashful with wd40 or the likes as you go along...
I have found that in most cases to just leave it be unless you have a substantial amount of Carbon and even then you are limited as to what you can clean without removing Pistons...
Good luck,
JW