So your saying that a huge build of of carbon from the exhaust was built up INSIDE the valve cover, like over the valves? YOU MUST post a picture of that, this would be the first time i have veer heard of such a thing! The only thing i can think of, If this is where it is built up, is that you have a bad exhaust valve seal and guide and its just pumping exhaust up into the valve area and past the CCV. When oil vapor or in this case strait up motor oil meats diesel exhaust, it turns into a messy pile of hell. Seeing as newer fuel has solved a lot of this issue, its more burnt oil than it is sulfur. in any case. post a picture before you do ANYTHING please. if it is like i think, your head is coming off, magnafluxed for cracks, new guide and a valve with a new seal, and thats if your lucky.
the only way to clean it, is to take supper high care nothing falled down into the pan, i would pull the oil pan like Oil hammer said, it could be from piston ring blow by and its like that all over the internal down low. a usual amount of blow by is normal but to see anything up in the head is un head of, even in a old damaged engine, trust me, i have seen a few diesels with no rings left on them and lots of blow by so much it was blowing all the oil out the tail pipe.
you probably have not damaged anything yet, as its been running but you never know, until you get to the bottom of this, dont even try and start it. that much build up could ruin lots of moving parts as it could clog up the oil pick up or the filter, i have no idea, all i know is that this is unheard of, not normal in any regards and a sign of major engine damage to either a valve or a piston.