I'd not recommend much cleaning of piston tops for the very reason indicated -- abrasive residue in the rings. Just scraping the bulk of it off with a scrap of wood is fine -- the rest will burn off fast enough when you get the head on.
You will find out if there is any in there soon enough, I suppose.
If you've not overheated it and it has visible crosshatching, the rings are fine. If it has been overheated, the spring in the oil control rings can anneal, preventing the scrapers from removing the oil from the cylinder wall, so it will "pump" up into the combustion chamber.
There won't be any excess blowby in this case, as the compression rings are fine.
However, I suspect all the cylinders would be affected, not just one, and the amount of oil that can dribble down past worn-out guides is astonishing -- the head on my brother's 300D was actually dripping oil when we took it off. Quite enough to produce the fouling you saw.
The diagnosis here is that new seals only fix the oil consumption problem for a few months.
Best of luck -- it's a nice car when it runs, and I'm sure you will appreciate not having to fill the crankcase every other day!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!