I think very perverted few methods can work, as in unless your fetish is on ashes, the pile of ashe has to have some connection to what it used to be as a zako.
So work (animated or not) would have to show steps between flesh and ashes, frame jump between two with blips inbetween is going to be the normal 'cant raise ratings and cant have objects in the screen'. To satisfy fetish in general sense, I would imagine something that shows needless steps, in forms of like bone showing, skin melting, clothes torn apart, set on fire, and such. some sort of ryona/guro process to show this person will be ashes.
if not, id say sound, as that can be enjoyed with blindfold so being ashes has no issues. But yeah I think issue is for most disintegrated works, the 'normal' fetish people will just think 'are you fapping at the girls or the ashes?' so my point is there needs something that focuses on disintegration part to make ashe pile part less important. that would be main dish, and ashe pile would be napkin or ice cream afterwards.