Well, I've done something similar before on marine engines. It's a different color but basically the salt water gets into the plug wells, gets dried out by the heat and basically kind of electro-plates itself to everything from the electricity. Usually the result of a leaking/cracked exhaust manifold. Anyways, that's neither here nor there.
What I had to do was get a long skinny drill bit, a compressor with a blow-gun that had an extended nozzle on it (to get down in the well), a penetrating fluid, drill, and an angled pick. Safety glasses are nice too, along with a rag. But to each their own.
What you do is go to a well that is good, put the drill bit down into the well till it hits bottom, and using a piece of colored tape wrap it around the top of the drill bit so it lines up with the top of the valve cover. You have just made a depth-guage on the bit. You know if you drill down to where the tape hits the valve cover, you are drilling into the head. Go ahead and chuck the bit up into a low-speed drill, preferably a light-weight one so you won't get tired since this will take a long time to do. I would guess an 1/8" or smaller bit, but you will need to find one that is 6" long or so. Just get whatever is cheap. You are basically going to destroy the drill from a quality/precision standpoint so there's no need to get anything special.
Here is what you want to do. First flood the surface with your penetrating fluid. Don't need to fill the well, just make sure the surface is covered all the way around. Using the drill, drill into the stuff about 1/4" or so, then move over and drill another 1/4" deep hole next to it. Keep doing this all the way around the plug until you end up back where you started. Try to keep the space between the holes as close as possible. If you can actually drill one hole easily into the last one then it will be quicker and easier. Now take your pick and see if you can knock the wall between the holes out. If you can you will end up with what looks like a groove all the way around. Now take the blow gun and blows all the loose/dirty crap out of the well. Put a rag over the well when you blow it out to capture most of the scarf, and glasses will help you look in the well while blowing it out. If you start on the low-side of the plug, you can just spray in your penetrating fluid as you go. that way you always have a clear view of where/what you are drilling.
Once you get within a 1/4" or so of the bottom of the head, you want to go very slowly. The best option is trying to crack/shatter the last bit so you can pull it out without drilling it. You can also try breaking the plug loose as that will sometimes crack it all up. Make sure all the penetrating fluid and garbage is blown out first. Just stop if it doesn't start to move with a normal amount of force. Try breaking the non-corroded plugs loose to figure out what normal force feels like. DO NOT remove the plug if the well is full of crap! If you can get the plug to break free, screw it back in and blow the well out really good. Once you are sure there's no loose crap in the well, you can take the plug out and use a plastic cap or bolt or cork or something to cover the hole and give you more room to work the rest of the garbage out.
Now, for the disclaimer. You can seriously damage everything by doing this. If you get impatient, and go hercules on the drill, you can drill through your valve cover, head, etc. If you are impatient with the cleaning part, you can end up flooding your engine with whatever this grainy crap is and penetrating fluid. If you aren't lining up the drill right, you can destroy your gasket surfaces guarantee'ing you will need to swap the head (and probably covers).
But, that is an option and will work if you are very careful. If it is not oxidization and really a resin/epoxy then this might be more difficult.
