Paul,
Not sure if these people supplied the sealing compounds but this link **LINK** gives a good overview of what might have happened to your friend's cylinder head – and rather confirms my thoughts that mechanical rather than chemical means will be the only solution.
I spent a merry few hours with small drills, bits of steel and compressed air clearing the blocked oil way which had left one of the cam bearings and two of the cams without any oil. The first time ity happened, I was in a hurry and just replaced them, the second time it happened rather more rapidly and that was when I found the vaguely crystaline, semi translucent material.
On the plus side, the original problem occurred in one of the cylinder combustion chambers. Apparently there was a batch of heads where the inner core moved allowing a very thin chamber to water jacket wall which subsequently developed a hole. A fast run back from Germany resulted in the combustion products gas-axing their way into the water jacket and limping home on 7 cylinders.
The cure was effected by a friend of a friend who had deep experience of welding heads for dragster and he repaired it but left it with some porosity or tiny holes. A company on Slough Trading estate was recommended and they successfully applied the pressurised compound (from memory, it was a resin). Whilst I was there I saw a pile of different bell housings and blocks from 'one' of the F1 teams.
The fix has held for another 100,000 miles plus over at least 15 years – probably more…not bad for a bodge worthy of JS…..