Mildman, in january i took my engine to a workshop and they completely re did my engine: hone, bore, decked the whole shebang. BUT! When i pulled my head it did have detonation marks and carbon residue as you have described in your own instance. I replaced 2 out of 4 pistons (1 cracked, one splattered !) but the other 2 i used degreaser, a bit of petrol, rags/soft wire brush and rubbed quite hard until most residue came off.
I got it down to faint black lines/splodges around the outside of the dish but thats about it.
Depending on how old your pistons, some are past their used by date and are not salvagagble to look pretty and clean again! A got a set of 4 cast ACL pistons for $105 so its worth getting some new ones. This includes pins aswell.

The deck it wasnt too bad but did have gasket marks/residue on it so i chisled these off carefully and for the big bits carefully used a very thin grinder disk to get them off without touching the deck at all. Came up as new and didnt have ANY scratches or dents from the cleaning process.
It doesnt need to be shiny new, infact im sure a bit of roughness mite sufice well for the gasket to imbed itself into, mite not but im just
guessing here!

Anyway let us know how it goes!

Cheers
Dave