Having now given up on Goldtec, I've dismantled the wheel and had a really good look at it. It's caused by galvanic corrosion between the s/s spokes and the alloy shell. On the LHS, the outside spokes have really eaten into the shell, spalling off the black coating, and it was about to fall apart in several places. On the RHS, the oil spatter has protected it so (after wiping off the sludge) there is barely any corrosion, the coating is largely intact, and the spokes haven't made much of an impression on the flange at all.
No-one now makes a 135mm hub with 36 holes and a wide enough chainline for an MTB, so I'm going to have to fit something different or buy a 32h rim and new spokes :-(