Try some mineral spirits (AKA paint thinner). The cured powder coat won't be affected by it.
I just used some to clean up the wheel weight tape residue on my wheels and while I was at it I attacked some of the more persistent stains that the WD-40 has not been able to get off the wheels. Here too, no negative effect on the cured paint.
Just be careful not to get excess solvents on the clutch release bearing. They may be able to seep through the seals and degrade the grease in there over repeated cleanings.
I used to use brake parts cleaner to periodically clean out the mixture of dust and leaking oil in my clutch, but I got suspicious of the release bearing because of my solvent use. I put in a new bearing from Chris at CA-Cycleworks a few weeks ago and gave up aerosol solvents around sealed bearings.
I haven't tried a clutch pressure plate, but when my gold colored brake calipers got dirty from brake dust, I was afraid nothing would make them look clean again. The first time I tried Simple Green on the bike, I was amazed at how well it cleaned most of that brake dust right off. A few minutes with a toothbrush in the crevices and it almost looked like new.
You might want to see if Simple Green helps. Just be sure to rinse it off thoroughly.