The walls of the cylinder bores were solid (and clean until some coolant spilled in). I'm fairly sure that the coolant did not enter around the cylinder walls.

My only real concern is that the pith fell on top of the lifters and would likely go somewhere before it got down into the pan.

Also, I was checking the head out last night and some of the valves have residue on them. Should I at least clean this gunk off with a wire brush or scraper before putting the head back on? Some of the gunk looks like soot, the stuff on the #1 valves looks a little more like sooty ash. #1 is where I had almost no compression when I tested.

I would think the valves and top of the combustion chambers would be made of something strong enough to handle a wire brush/wheel - but I don't want to make too many assumptions about this part of my engine.
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1967 P220 (My every-other-daily driver...), 1971 P1800 (The Parts Car), 1972 145S (1993-1997), 1977 245 DL (1993-1999), 1983 245 (1998-2001), 1986 745 GLE (1997-2005), 1990 745 GL (2003-4), 1995 945 (2005-Present)....